Perhaps pick a target turn. If you prefer an average of 10 turns per game, roll at the end of turn 9, a score of 5+ ends the turn. Same on turn 10. If the game gets to turn 11 definitely end the game. You might even play turn 11 with rules for fighting in twilight. Though throwing in rules changes at the end of a game sounds like a bad idea. Never mind.
It does need to be integrated into the rule system. Our current WWII system is:
+1 each turn, plus
both players roll a D3, add the difference (+0, +1, +2), plus
+1 for significant event during a turn (currently, air strike or off-table artillery bombardment)
Battle ends when the total exceeds the days in the month the (campaign) battle is being fought in. We use a TFL-style card activation system in our home-brew rules. At first it looks too random with potential totals of 5 (or less) to 7 [the pack contains a card for each player to have an airstrike or artillery bombardment]. However, nobody is going to forego an air or artillery strike, which is essential to press home an effective attack, and retarding the passage of time if they do not arise or accelerating it when they do happen keeps the tempo of the game humming. It fits our requirements but would probably not do for many other rules without significant adjustment. Because time advances during a turn it also prevents last turn issues as the day may end with a dusk strike just before the attack goes in, much to the chagrin of the player making an attack. As you cannot be sure of card order you have to get on with your attacks less dusk fall before you have concluded matters.
We have currently moved over to the seventeenth century. Using days of the month is good for February but makes January the same as July so I am experimenting with hours of daylight for the target and variants thereon. The key is to define the range you are looking for and finding a methodology that will deliver it and ensure some ambiguity. I think ROMMEL is a great set of rules BUT by fixing a day at 16 turns it can give a very bad game-end with a lot of last move play twisted to achieve the objectives. I find this very annoying and a blemish on a very good set of rules; anyone found a good fix?