I once felt certain that there was some sort of undue drain on the battery (of indeterminate age) in my car, went to a garage where I was asked to leave the car until the next day, I left it with a second battery which was new. When I returned I was told both batteries were useless - they had been connected to the car and drained - proof was the low voltage at the end.
I did my own rummaging around and disconnected a suspect wire. Two years and more the old battery is still in the car doing fine, the other will perhaps one day be installed (if the other finally actually fails).
I had the advantage of a battery charger of my own - these are not that expensive and really worth owning. First check I would suggest is get the battery charged up (preferably out of the car), then reinstall it and every time the car is to be parked for more than a few minutes, disconnect the battery.
If it starts the car for some days then the car is likely unduly draining it when connected. If it still goes flat quickly then the likeliest cause becomes the battery itself. Another way is to thoroughly charge it up (disconnected) and then leave it disconnected (for example overnight) and see if it will eventually start the car - if not, the battery is likely poor.
It is also important to consider the possibility that the alternator is failing to provide a useful charging current.
All the above assumes that disconnecting the battery does not cause undue complications with assorted electronics in the car. As stated already, good contacts are important or else the car will not quickly charge up the battery - and two hours may not be enough if the battery is already flat.