I would base the time allotment on the number of miniatures controlled by the player - and give more time for powerful minis with special rules.
I think in Space Hulk, basic allotment was 30 sec pr. squad, + 30 sec per sergeant, and +1 1 min for a captain.
As this is not a boardgame, I would suggest allowing a little more time - maybe 1 min per mini, with extra time for the 'rules-heavy' ones.
I'd also suggest using a stop-watch, so that time can be stopped if need arises for consulting rules and the likes....