Put very simply, it takes time for a general to decide what orders need to be given, to issue them, for them to be transmitted to a subordinate, have him decide what needs to be done to implement the order and then issue his own orders and so on down the line. How much time? Well, it depends on a lot of things including what level of game you are playing. Since the games I intend on playing will usually have 6 to 8 regiments, the orders usually won't have more than 500 or at most 1,000 yards to go and will be issued to either a Brigade commander or directly to the regiments, possibly in person. So perhaps as little as 5 minutes might be taken up with orders and perhaps another 5 minutes to prepare to execute them and get under way, less if the units being ordered were already prepared and just waiting for the word.
Once under way, the troops will progress at at least 50 yards a minute, usually faster but there are often little delays so lets say 50 yards. Typically they have started within 1,000 yards of the enemy and often from as close as 500 yards. So between 10 and 20 minutes are taken up with the approach march. Resolution of the attacker will rarely take longer than another 10 or 20 minutes. Taken together, the time taken to issue and prepare to execute the orders will be as long as the time taken to close with the enemy and as long as the combat takes to happen.
There was a time when written game orders and short turns with multi-turn combat resolution (at least for firefights) were in vogue (eg WRG 3rd ed Ancients, Grant's The Wargame). Usually these orders were changed by a courier with a minimal loss of time though I have played several rulesets where there was a chance of misinterpretation and delay (eg Bluebear Jeff's Tricorne rules that I had the chance to enjoy a game of a few years ago, see also the last issue of CWJ).
The older rulesets called for orders to be written in realistic terms and left it up to the player to interpret them in good faith. Some rules also tried to impose a need for players to justify issuing a change in orders. Reaction by subordinate commanders to local conditions was always a matter of contention leading various rulesets to adopt reaction tests or start specifying official orders such "Attack", "Hold" each of which placed obligations and limits on what units could do (Shako is an example of this approach).
One way to avoid the question of orders is to lengthen the turns so that the order, preparation, approach and combat all happen within 1 turn, with the possibility of the combat not being fully resolved. (Volley and Bayonet is an example of this approach). There is a lot to be said for this, especially when playing large battles but for smaller battles it can result in a very short game with few player decisions being made and battle resolution potentially resting on a handful of combat resolution dice.
So what about something in between? Lets say the process was that this turn I issue the order and the unit can change formation and perhaps shuffle a bit to line up, next turn the unit moves up to 500 yards and into contact with the actual fighting happening the turn after, assuming the enemy was still there? If playing an opposed game, that would introduce some interesting challenges, in much the same way that Charge! does with its long moves and written turn orders. You might see a courier arrive and the enemy prepare to move, but where is it going? Played solo however, there would be no mystery so one would have to rely on role play or some form of AI, reaction charts etc to determine the other side's reaction.
On the other hand, you could assume that the order phase was taking part behind the scene and just make the moves long enough that there is not quite enough time to fully react on the other side, and possibly not enough time to safely change one's mind, and make the combat results drastic enough that 1 turn of combat will sometimes be enough to resolve an attack. It seems to me that this is how many of the older sets operated whether using simultaneous or alternate moves. (eg Morschauser, Featherstone's rules in BWMS, Charge! ) This approach also reduces the need to fiddle with units to edge them closer to where they might be needed and allows an attack to wait until an artillery bombardment can be made rather than needing to start moving right away in order to make it into contact before the game ends.
One other approach to add texture without shortening turns that I have experimented with in the past, is to lengthen the moves but have a multi-phase combat resolution. This was inspired by Simon MacDowall's Comitatus where units halted at bow range, there was an exchange of missile fire then units whose morale held closed to javelin range (if desired), then exchanged shots again and finally, charged into contact if morale held. This shows some of the detail and flavour of the combat without allowing players to change orders every 5 minutes.
Fire and Fury sums up this approach well by having long moves then resolving combat by having defensive fire then offensive fire then melee.
So where does this leave me?
Pouring another coffee and contemplating the game laid out on the table upstairs.