In game design, timing, and especially tacked timing of "combat rounds" and for chases and things like that, can be very important. This article will give at least a brief treatment to some of the considerations that go into round length in a game.
Savage Worlds and Warhammer both also use about 6 second rounds of combat. The PC turns are effectively the same 6 seconds, but happen simultaneously, with the initiative order signifying really only a fraction of a second difference to put one person above another "in line", so a turn is 6 seconds, and a round is made up of multiple turns, but it also is 6 seconds, because they overlap.
Many if not most games usually include at least a brief note about "attacks" or "shots" related to round length, stating that a melee or ranged attack is not one single sword swing or firing of a weapon, but a brief cinematic shot or scene of an exchange in combat, complete with dodging, parrying, counter-strikes and such.
You can see where it would be nearly impossible for characters to really keep track of their ammo, especially in the heat of battle - so think of every ranged attack you make as not a single pull of the trigger or loosing of an arrow or bolt, but as perhaps up to half a dozen or so, in the span of 6-10 seconds (depending on game system).
Rounds are the nebulous, uncertain but nominally 6-second (usually) length they are in most games because it is a contextually short amount of time, but long enough to be able to "hold" most significant things or actions that combatants would do.
If you are truly serious about "getting it right", the best way to understand round length and compare your own ideas are to simply do some personal experiments.
Pick up a short broom or other weapon-length item, hold it to your side with one hand as a sheath, and try drawing it a few times with the other hand, mentally or even using a stopwatch (or having someone else do it), find out what your average time is to "wield" or draw the weapon. Now do it with a watergun or capgun or bow (or that same sword-proxy), etc. You can get an idea into what kind of "window of activity" a large range of actions would fall. Take your faux-weapon and make a single strike, or even just punch or kick something (insert warnings, disclaimers, etc) and get an idea of how long these things take.
Now complicate your attacks by adding the time you have to take to aim at a certain object at a certain distance, to reload, to draw an arrow from your back or hip-quiver, raise your bow into position, nock it, aim and fire, or do an impromptu bit of LARPing or "shadowboxing" in your melee attempts, making parries, ripostes, recoveries, changed strategies and tactics, moving footwork.
Lastly actually walk at your normal pace a certain speed or amount of time, noting it, then determine any other factors not known at the time. For me it took about 7 minutes to walk around a quarter-mile (1,320 feet or 400 meters) high-school track, which worked out to about a 3 feet (0.9144 meters) per second (2 mph or 3.22 km/h) stroll, which is my everyday walking speed - that translates to 18 feet per turn or round, roughly half the standard movement rate of 30-40 feet per 6 seconds (so standard speed for a Movement 6 character in a 6 second turn game moves 4 mph on average).
From this you can determine what you WANT to allow for your game, in a certain span of time you're calling a turn. From a more concrete game design standpoint, it isn't so much that a round is "as long as it needs to be" and more "it is as long as the GM wants to allow it to be, based on how many actions he wants to be possible in a turn". A single punch or melee weapon strike could take as little as less than even a second (Bruce Lee and many martial artists have been documented as being able to strike with fist or foot, faster than a rattlesnake strike (90 frames of video, at 1/500 of a second each, or 1/5 of a second), which puts trained, top-notch melee unarmed fighters as fast as 0.10 seconds, or even faster.
You then have to "work backward" from these, and see what time frames can meet and average out, to allow most of these actions to all fit within the same duration, which becomes your turn and round length. This is where you have to make the judgments calls on whether or not you want PCs to be able to draw a weapon and attack without it being a multi-turn action, or drop, dig through and draw an item from a pack, etc).
For example, let's say in a game I'm working on, I've decided that the turns are short, encompassing only a single strike, but with the setup, lead-in, strike and resulting changed stance and location for both combatants, or a complex grappling maneuver, and possibly some posing - this still is well within a 3 second window, and that timeframe keeps popping up in my personal project, so it is probably going to be my base.
I don't have a lot of the other examples above in my game, such as drawing items from pouches and such, so I just give a penalty for "long actions" rather than making them multi-round, so I have no need to use the 5 or 6 second turn.
I've also watched some fight scenes from various movies, though mine were mostly martial arts/action movies (Jackie Chan etc), and most action/fight scenes came out to take a minute and a half or two minutes, from start to finish of the entire fight scene. That's for "non-Boss" fights, even with lots of stunts and choreography, while a "final showdown" with a major villain tends to clock in around 5 minutes. From this, I have the additional option of taking the non-Boss fight scene (1.5 minutes), and dividing that by the number of rounds I feel is a good average, or one which I wish most fights to take, to arrive at my seconds-per-turn idea.
If a simple but clever setpiece fight scene lasts 2 minutes, and I intend for the PCs to have to have the opportunity to attack 5 times in that duration, before the foes are eliminated, I divide 2 minutes by 5 attacks or rounds, arriving at 2.5-3 attacks = 1 minute, 1.5 attacks = 30 seconds. I hope you can where I'm going with that. Also note the preceding example is for mostly unarmed and fact-paced martial arts scenes, not epic fantasy brawling, but the ideas remain the same.
