20 amp circuit breaker trips after a few minutes or sometimes hours later
If you're getting a plain breaker trip, the usual reason is overcurrent.  You have too much stuff on that circuit.  

From your comments, a GFCI outlet tripping is not your problem.  

You can get power monitor devices that will tell you what each device draws.  The simplest is the Kill-a-Watt, which for about $20 and measures a single load. They make more sophisticated monitors too.

The breaker that's tripping has a number - either 15 or 20.  That's the maximum amps. Multiply by 120 (i.e. 1800 or 2400), that's the maximum watts, and also the maximum VA (volt-amps).  

By now you probably know which devices are on this circuit, so check every one with the tester, one by one.  Check it in the same conditions that make the breaker trip.  Tally up the amps, VA, and watts.   

At this point, people often find a big surprise. I can't guess what your surprise is.  Now you work the list, same as you would if you were trying to be "green" or save on your energy bill.  Lots of people with those motives will help you with this problem. 

FYI 1 watt of energy, run continuously, costs you $1/year or more.  Some upgrades will pay for themselves.  

If stuff is running continuously, you want your breaker capacity to be 125% of the continuous loads.