It probably depends what your goal is, like whether you want to simulate tournament play with time controls, or whether you just want to get familiar with solving tactical situations until you start to build up some proficiency with them. I know from experience that puzzles take me a certain amount of time (usually 10-15 minutes at most--30 minutes is an awful lot!) so if I find myself still totally stumped after that time I usually give up and look at the answer. One benefit of limiting time like that is that a fair number of puzzles have misprints (missing pieces, wrong side to move, etc.) so those are literally impossible to solve!
Another practice I've found that helps is to go through the puzzles at least a second time to reinforce my memory. It feels good to be able to look at a puzzle and recognize the solution within seconds. Imagine if you never looked at an opening a second time, for example: you'd soon forget about it, which isn't good practice.
Here's one puzzle I found in two different puzzle books. The solution is extraordinary and involved a brilliant trick I'd never seen used before. The book author even wrote "White produced the following brilliancy: ..."
Palatnik, Sam, and Lev Alburt. 2013. Chess Tactics for the Tournament Player. New York, NY: Chess Information & Research Center.
Supposing I have a puzzle book. Should I allot like 30 minutes to a puzzle and if I cannot solve it move to the next one. Or check the answer. Or spend more time. Or never look at the answer?