How much time you spend on one puzzle? I know solving tactics puzzles is one of the recommended ways to improve in chess. I spend around 30 min solving puzzles. 

How much time I should spend on one puzzle? 
During games we don't think 15-20 min on one move (of course depends on a stage of the game) So I though same should apply for solving puzzles. 

Would it be better for growth to spend max 5 min or better to to try to solve puzzle even if it takes say 20 min?
You should try to solve until you find the solution. If the puzzle is hard and has concepts new to you, it will take lots of time. 

I remember that I tried to solve tactical puzzles for days, and eventually solved them. If you solve yourself, you can be sure that you understand the concept, that's very important. That is the thing what makes you progress. If you try to solve for a few minutes and then give up and check the answer, your time and the puzzle will be probably wasted. 

If you think its too hard to solve in mind, you can set up a board and move the pieces. As long as you discover the solution yourself, your efforts will be rewarded.