How long should you spend on a puzzle before giving up?

If you are trying to solve a puzzle but don't think that you have found the right answer, and what point is it a better use of your time to move onto the next puzzle? Either by playing the best move you found or by choosing a different puzzle.

It doesn't matter, just follow your feelings. Anything you continue doing under mental duress is bad for you. If that means resigning every puzzle after 10 seconds then puzzling is not for you - perhaps chess is not for you. It's supposed to be fun! However, do look up the solution and try to understand why you couldn't find it.

Also note that the number of chess puzzles is virtually unlimited. There is nothing you need to learn from one particular puzzle, today. Opportunities to get it will return, possibly at a moment you are better prepared for it.

I forget which book it is but I read one written by a GM who recommended spending 20 minutes analysing each position in his book. Generally I either know or I don't after 2 or 3 minutes, so spending the extra 17-18 minutes would be less useful to me than seeing several positions and solutions in the same amount of time.

There is a massive difference between studying positions from a book and solving puzzles on chess.com. The positions are probably learning material and required knowledge in terms of the book - e.g. to measure your progress. Chess.com puzzles are commonly more general and they wander here and there. It is easier to switch from random puzzles than to leave the plotted course of the GM. In #1 you didn't give any information about the context of the puzzles so I assumed a general tactics training. When there are specific directions you should follow them unless you understand why they are not suitable to you. 

The original post did mean general tactics training like the chess.com puzzles but then I made a connection with something I read recently.

The book is Lev Alburt's Chess Training Pocket Book. There are 300 positions in that book.

I don't know the book but chances are that the puzzles are organized by subject. When you skip all the puzzles in a chapter because you didn't get the subject then you won't get another chance (with this book). Chess.com is not ashamed to revisit its challenges and its database is replenished with new ones regularly. So you need not fear that skipping a puzzle will lose you the opportunity to train on any puzzle theme you missed the first time!

All assuming you looked up the solutions of the puzzles you didn't complete to get at least something from the attempt!

In a real game you have to play a move too.
If you have not found the right answer, then this is a very interesting puzzle for you that teaches you something you did not know.