I go once per encounter, unless something big happens. If something utterly disrupts the situation, say an explosion or earthquake, then we reroll and pick up form there.
I run an "all comers welcome" college game sponsered by my science fiction club. I sometimes end up with 11 pcs at a table, and other times with 11 or more party members between pcs, cohorts, and temp allies. These are invaribly the times when the pcs decide to pull a multiprong assualt on the castle, or take on the entire column of orks. I've used several varitations on initive for handleing these.
1) Assult on cliffside castle.
PCs spilt into airial insertion and basement breech teams. Both teams tried to "quietly" enter at the same time. The flyer's and familiars scouted the open windows, courtyards, and rooftops, so they knew approximate locations and numbers of gnolls in each visible room. I ended up using 4 battle mats and a chaulkboard to map what was effectivly 6 levels of castel. When both groups were in position, I had the pcs multiply thier init mods by 5, and roll initivitive on d100. Badbuys rolled as needed. I expected some breaks in the action, but somehow one squad or the other was always fighting. We stayed in initive for 6 hours of real time, and maybe 25 minutes of game time. There were some odd instances of one group looting while the other was fighting for their lives 3 floors below, and then the upstairs boys fighitng the oger while the downstairs guys where killing sleeping gnolls under cover of a silence spell. Then the upstairs people got bottlenecked on the stairs trying to get into a room with 50 gnolls, while the downstairs people opened the other door to the same room, and were bottlenecked trying to get out of the gnoll sleeping quarters where their spell casters had to wait out their own silence spell while the hafling ranger held the gnolls at the door.....good times...
2) holding the recaptured fort.
Different game.
PCs had captured a fort. The fort was expecting reenforcements. Pcs decided to ambush them form within. 11 party members, 140 orcs.
This time I took prtty much every mini we had, and put the orcs in rough squads depending on which minis I had. Orc orcs, kobold orcs, gnoll orcs, chicken orcs, dog orcs, little green army man orcs.......Used the same d100 type initive roll, but used a sqaud roll for each group of orcs. Orc commanders, casters, and seige trolls got seperate rolls. I modified the encounter to match some of the mageknight minies we had, so some of the orks were riding spiders and lizards that I decided could climb walls. 4 hours of grind later, the pcs were down to the last and largest orcs on warbeasts, and orc casters. I had been pretty rough on them (orks on dire bisons bursting though the overturned wagon blockading the door, cr 4 barberian on cr 8 war rhino, the lizerd mounts could breath fire...pcs level 4-2) so I used the elf calvery escape hatch. Worked pertty well as far as maintaining the flow of the battle. The rebuilt wagon is now considered a party member.