I am new to this also. If I am with someone who decides after 10 minutes they are ready to move on, I will not log a DNF. I, myself, will stay for over 30 - 40 minutes +; if I cannot find the cache, I will record a DNF. Also, if there is a natural impediment - water due to rain, snow, etc. I will not log a DNF - I will go back after a while and try again. I have also emailed the cacher for another clue if I cannot find it.