Obviously it depends on what exercise you're doing, what level, what the goals are...there's a lot of stuff to consider here besides "the burn". I know from your other posts that you're having trouble with difficult strength that other girls don't complete, so you're worried you're doing too much strength. My coaches always say things like "no such thing!" to a statement like that. I think strength should be hard enough that you're getting strong enough to do the skills that are required of you (think bars, especially). I don't have to do toe on-hecht-kip handstand-giant-double back, so I don't have to be as strong as the girls who do that. I'll probably never be able to do that, but I do have to be strong enough to do a kip handstand-giant-regular flyaway. If I'm getting stronger and I"m getting my skills, yippee! If not, there's either a problem with the skill progression, the coaching, fear, or conditioning.
I think my strength this year is a little bit easier than last year - new coach. Our old coach was Russian...very Russian...we did strength just about constantly: we had strength at the beginning of practice, strength in-between turns on bars (lots), some strength before beam, strength between turns on floor, and handstands at the end of practice. Level 5s and 6s. Now we have strength at the beginning of practice and some on beam and bars, but it's a lot more drills-focused. Harder exercises, fewer reps, and more about body shape and stuff than straight strength - also I'm stronger, so it's easier! Takes less time, too, and I used to stress out more about strength.
I guess in terms of strength the questions you have to ask are: 1. Am I getting stronger? 2. Am I strong enough for my short-term goals? 3. Does the strength we do leave enough time to work on skills as required? (You need more time during competing season to work routines than condition, obviously).