My experience finding my 5, 10, & 15 rep maxes was that it wore me out. And it took forever. I spent about an hour each session.
Why did it take me so long? I had no idea what my maxes would be. I had a pretty good idea what my 1 rep max was on the 3 powerlifts: deads, squats, & press - but I had never tested my maxes on higher rep counts. After a couple of warm-up sets, I tried to hit my max within 3 sets. On some, I never reached my actual max, but called it a day and guessed how much more I could lift (I couldn't use up all my time & energy on that one lift). I failed on some and guesstimated how much lower my max might be for those.
Why did it wear me out? I started HST after spending 6 weeks training for a 5K - so I pretty much only ran for 6 weeks before testing my maxes. I was a bit deconditioned. The testing was brutal for me because I was pushing my muscles to their limit for every lift and they weren't accustomed to that sort of thing. I wouldn't say I was 'tired', per se. But my muscles were definitely cashed at the end.
You may be different, though. Maybe your muscles are better adapted to this sort of effort. Or maybe you didn't push yourself hard enough. It is really difficult to tell, honestly. I would assume that most people feel like they got a good workout in when testing their maxes, since you are testing to find the upper limits of your strength.
