Up to which point should you exercise?

Factual Questions
1

Every time I tried searching for an answer on how long you should exercise, I got the generic 30, 60 or whatever minutes answer. That obviously isn’t the right answer, since you can do both very little exercising and not actually feel tired and accomplish anything or you can also waste all your energy in the first few minutes, so doing 10 minutes of one exercise may be equal to doing 30 minutes of a easier exercise,etc.

So, in my example, I am very skinny and can do little, but I can do for example 20 or 30 push ups and then I can’t do a single one at all, however if I just wait for 2 to 5 minutes after doing them, I can do another 20, then can’t do anymore, wait for a few minutes and 10 or 20 again,etc.

So at which point is my exercise complete, when I can’t do anymore even after the short pause?

2

You mentioned that you are skinny, so is your goal to add more size? Generally speaking, more volume equals greater size (up to a point, of course.) Here’s Brad Schoenfeld on that topic:

http://www.lookgreatnaked.com/blog/how-many-sets-do-you-need-to-perform-to-maximize-muscle-gains/

You will be able to pull off getting more volume on pushups if you don’t go all the way to failure on your first set.

I have had success years ago with bringing up my max amount of pushups using the following program:

When working out with weights, I do like going close to failure on every set, but I can stay in the rep range I’m aiming for by lowering the weight on subsequent sets.

3

So to simplify it, I should get close to failure with every set, but not entirely, like 70, 80% of my potential for each set? So for example since I can do 20 to failure on each set, I just do 15, then rest and again the same thing?

4

You might want to look at training zones and thresholds, aerobic/anaerobic fitness, VO2 max, all the good stuff. Random starting point: Heart Rate Zones

5

There is no exact answer, but that sounds like a decent way to get more volume without dedicating too much time to pushups. Then try going close to or to failure on some days, or on every pushup day on the last set.