For me, Surprise is often all or nothing. Either nobody notices and the bad guys pull off their ambush, or at least one good guy notices and starts alerting the others to danger.
In the middle of those two is about one round where the bad guys KNOW they have been outed and decide to attack immediately. Maybe conditions were not right for a perfect ambush but they are going to lose the element of surprise if they wait any longer.
There's also the case where some enemies are hidden the whole time, but others are visible. Maybe negotiations with the orcs is going south and a totally unseen sniper catches the signal to start firing.
I've seen DMs rule on either side of the fence for that last example.
In the middle of those two is about one round where the bad guys KNOW they have been outed and decide to attack immediately. Maybe conditions were not right for a perfect ambush but they are going to lose the element of surprise if they wait any longer.
There's also the case where some enemies are hidden the whole time, but others are visible. Maybe negotiations with the orcs is going south and a totally unseen sniper catches the signal to start firing.
I've seen DMs rule on either side of the fence for that last example.