Probably a bad breaker, but the symptoms are also consistent with a mild overload causing the breaker to heat up and trip over time, the longer you let the breaker cool before resetting, the longer it takes for it to heat up and trip again.
I'd leave it turned off until you've ruled out a short in the wiring or other overload condition causing it to trip - if it's a short somewhere, that short is causing a lot of heat somewhere in the walls or other inaccessible place.
It's possible that the wiring is not straightforward and there's a load connected to the breaker that you're not aware of - like maybe the toaster in the kitchen is on the same circuit so be aware of what other appliances in the house may be in use when you experience the trip.
The hard thing about finding a short circuit is that if it's due to, say, a bad connection in a junction box under the floor, it may not short out until you walk near it and make the wires flex enough to touch. If you notice any kind of burning smell associated with the trip, keep the breaker turned off until someone finds the problem.
Some types of breakers are subject to "nuisance tripping" even without any overload or wiring problems - if you have an AFCI or GFCI breaker, you could be experiencing nuisance trips due to a device that's plugged in.
If you have 20A service to your bedroom, you're probably in a newer house (15A is more typical, especially in older construction), so a short is less likely than if you're in an older house so the most likely cause of the problem is a bad breaker. Hopefully the landlord had his electrician swap out the breaker since a breaker is relatively cheap compared to having the electrician make another service call.
The consequences of a short somewhere are severe enough (i.e. a fire in the walls or other inaccessible area) that it's worth ruling out a short before you leave the circuit turned on. If the landlord can't fix it to your satisfaction, hire an electrician of your own and bill it back to the landlord. Even if it's just a bad breaker, the landlord is responsible for wiring problems.
The fact that it stays on after waiting 5-6 hours probably means that it's not a short, but I wouldn't dismiss any repeated circuit breaker trips since the circuit breaker is your only warning about a dangerous short in the wiring.
Best Answer
Ironically your wiring system has reached 30 years which is its half life. Meaning from here through the next 30 years an average system can experience 50% failure of it's components. This is just a statistical average and does not apply to any one system.
I would start by performing some maintenance on the panel. First I would remove every conductor from it's panel connection, including neutrals and inspect and clean the conductor. Then with a voltmeter carefully turn it on and check to see if you are getting 120V on each pole of the breaker to the ground bus. Turn the breaker off and reconnect the circuit, turn it back on and recheck the circuit to make sure your voltage is OK. Then repeat all the way through the panel. I would start with the main breaker and go all the way through.
Second as long as I already had the panel open I would remove each breaker and check the resistance through the breaker with an ohmmeter. the resistance on all breakers should be consistent throughout the panel. If I experienced a higher resistance than normal or an open circuit, I would replace that breaker and I would check the connection to the bus for any corrosion, coloration or arcing. I would also do the same at the breaker conductor connection.
Then once I have reassembled the panel. Perform a voltage check by measuring the voltage from the main service feeder to the ground and the neutral, and check it against the voltage between the breaker post and the neutral/ground to make sure it is consistent with the incoming service voltage. It should not vary much more than one volt.
In short you are indicating that the panel connections are loose and it is not just with one circuit. So lets clean it up and see if we can get it back to proper working order before calling in someone for a complete panel replacement.
Good luck and stay safe.