I kind of like what Caio has to say about strength and conditioning training. I think if your only training goal is to get better at jiu-jitsu, then any time you are doing squats or push ups you aren't making the most of your time. If you want to increase stamina and strength for jiu-jitsu related purposes, then do exercises that have the added benefit of increasing muscle memory for specific moves. For example, if I just wanted to work on my abs I might do leg raises, but if I wanted jiu-jitsu abs, I'd do as many triangles as I could for 3 minutes. The first one isn't a waste of time by any means, but the second one is jiu-jitsu specific, and therefore more desirable if you're only working out to improve your jiu-jitsu game.