It really, really, really depends on the stakes you're playing, and the kind of place you're at.
If you're playing at low stakes, you can quite possibly get away with anything and everything. Unless you're in Reno, where even $50 is a big bet for some places.
And your general operational approach to the game can effect things. If you're in Vegas, keeping sessions short, not giving your name, and moving from place to place, you can get away with little to no camo, mainly because there's not enough time for them to really gauge your play.
To me, the most important thing about camo plays is to know which plays you're going to make, and how often you're going to make them, in advance. Some plays can be very expensive, especially if you have a big bet out. Failure to split eights, for instance, could be very costly. Standing on 16 vs 10, in contrast, isn't so bad. Blackjack Attack and Burning the Tables in Vegas both talk about this concept of "cheap" playing mistakes.
edit: oh, your big question is when, and that's tougher. I'm just a neophyte here, but the reason it's tough is because the heavy-lifting for counter-catching at all but the tiniest places is done by surveillance. How do you know if they're watching you? You don't. But they might start watching you for a few reasons:
1) You're betting the largest amount in the pit
2) You have a huge buy-in
3) The pit critter requests surveillance on you after a bet jump
4) They stumble across you or are bored
Of course, if a pit critter is actively hawking your game, you know at least somebody is watching, but it might be almost too late, they might have surveillance clocking you as well.