Should progressive overload be done at every training session or should I do a few sessions at a certain weight/reps before upping things? Also how much should the weight go up in percentage terms?
Honestly this depends on quite a few factors.
For beginners, it will happen at every session with ease. With intermediates not so much.
Sometimes taking a couple sessions before adding weight is a great idea.
You see there are more ways to progress than just adding weight to the bar or implement.
Add quality, or rather train with same condition until you are better at that condition (you're alluding to this in you post).
In terms of the % of weight added, again this can depend on the same factor above, basically training experience. 2.5kg might be fine for bench for say a beginning lifter. But it might be too fast for an advanced lifter.
Also depends on how you like to train. You can take the bigger jumps for say 4 weeks, then drop back to week 2 and go again. Like a wave load that over time still progresses linearly. Or you can take the smaller jumps and just keep going until stalling. Then rest, drop back a couple weeks. Go again.
Same result in the end really.
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