Thanks to William for the great advice!  Directly answering your question:  you are done with a lesson when you completely understand the idea being taught and, or can do the physical skill being taught.  That doesn't mean you should be know it all & able to do it perfectly right away.  Keep going forward to learn new things & stay interested.   Keep going over old things as you gradually improve your understanding & playing.

That's when William's statement about balance comes in.  It's a very individual thing.  You want to keep moving forward to keep improving & stay motivated, but not so much that you get overwhelmed & in over your head, then discouraged.  Learning guitar is a constant process of repetition & refinement.  For as long as you play you'll keep playing many of the same things over & again to keep them under your fingers while you gradually add new skills & improve your old skills.

Recently you were asking about learning the musical alphabet.  And you were concerned you might not be understanding it all.  That's completely normal.  Try to grasp what you can & keep going forward.  At some point you'll get more info & on scales & it will gradually start to make more sense.  And the most important part is to keep playing, keep working on your physical skills.

And if you get stuck or confused again, just ask in the forum!

Hope that helps. Keep practicing!