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.
Since it's a Ground Fault Circuit Interrupter (GFCI) breaker, it's very likely that you have a ground fault.
GFCIs work by detecting an imbalance in current between the ungrounded (hot), and grounded (neutral) conductors. This is why you only see the trip when the circuit is under load. If there is 0 current flow, the current is balanced.
I'd start by inspecting the receptacle. Turn off the breaker, then open up the box where the receptacle is installed. Look for damage to the receptacle itself, or water/moisture in the box.
For outdoor installations you should always have a weatherproof box, and a cover that is weatherproof even when the receptacle is in use.
Best Answer
The two branch circuit breakers will trip if the load on their protected circuit is greater than 32 amperes, and 25 amperes respectively. However, since the main breaker trips at 40 amperes. If both branch circuits are pulling a full load the branch circuit breakers will not trip, but the combination of the loads will trip the main breaker (32 + 25 = 57 > 40 as you have said).
It's a very common situation to have the branch circuit breakers total more than the main breaker, but this is almost never a problem since the circuits usually don't pull a full load. Remember, circuit breakers are there to protect the wires. They shut off the power if you are drawing enough power to damage the wire, so installing a larger main breaker is likely not an option (unless you contact your power company and have the service upgraded, which will include upgrading the feeder cable).
Contact your local power company, and speak to them about a service upgrade. Other than plugging less stuff in, or cycling loads, there is not much you can do yourself in this situation.