How long to find a tactic?

I usually spend time looking for a tactic when using the trainer because, obviously, one exists. In a game situation, if I don't recognize a pattern, I struggle with deciding how much time to spend looking for an opportunity? I realize that experience will allow me to find them more often and quicker but in the meantime, what's a realistic time to spend if nothing jumps out and grabs you? 

I only play turn-based on line with three days for moves so I ave all the time reasonably needed but wonder when I should give up while knowing that maybe something is there and I just don't see it right away.

Approaches/suggestions/ideas?

You said it yourself, experience is key. the more you train your board vision, the easier it will become to find these tactics. The uncertainty factor can be quite frustrating (even to GMs) during a game. The best way to combat that is to also study positional chess. This will allow you to better assess what is going on in the position.The more you understand the game's  seven positional elements (superior minor pieces, pawn structure, space, material, control of key files & squares, development, and initiative), the easier it will be for you to come up with a plan of what to do, and the tactics will come to you a bit more naturally during a game. (From my experience)