I could have sworn that this was in AE...
i dont think we could ever link ambushes to a timer
you can link ambushs to most any added objective, but adding a timer is only a fail condition for the mish
Perhaps you could set two enemies (factions) to dislike each other a bit further down the map, one from the enemy group you're actually using through it, one from a dummy enemy group created for that purpose, stack the fight for the real enemy group and tie the ambush to the dummy losers getting killed or being at X% of HP.
It never existed but there used to be a way to get it to happen.
Once upon a time I had a battle between two enemies and then spawned an "empty" ally off of that battle's completion. (I call an empty ally an ally that is not required and that has no surrounding enemy group - so when it spawns it instantly completes)
When the battle ended and the empty ally spawned I would attach an ambush to that. It allowed for a moderate level of timed ambushery.
But as of i21, if I don't particularly engage in the battle, when the ally spawns it won't auto complete and therefore the ambush wont spawn. It's a bit annoying but as others have mentioned you can get around it. Particularly I just put a random ally boss somewhere on the map and have a partol of baddies form nearby him. They kill him and that spawns a "timed" ambush.
If you'd asked me this morning I would have said, "Yeah, you can set ambushes to trigger at certain specific times during a timed mission." I thought it was in there... but either I'm missing it or I was wrong.
....did a search and didn't come up with anything. This has been brought up before, hasn't it?
I have a comedy arc I'm working on now that has rebellious cooks in the Arachnos kitchens laying siege to Lord Recluse's dining room. His personal chef is up in arms over it and needs you to defeat them QUICK before Lord Recluse (the Hungry) comes in looking to kill everyone that's setting his din-din time back a notch.