Exterior lamp post trips breaker after a few hours
In order to troubleshoot your problem you need to have a few meters. First you need an amprobe. Check the amount of current your circuit is pulling at the breaker when the fixture is on. Let's say you have a 15 amp breaker and your amprobe is showing you are pulling 16 amps. A standard molded case breaker has two operations. One an instantaneous trip and a thermodynamic trip. Since you voltage is too low for an instantaneous trip. It is slowly heating up and then tripping out on overload over time according to the breaker curve of that particular breaker. If you are reading an overload on your breaker and the only thing you have on that circuit is the light post, then you have a high resistance fault somewhere in your circuit. I can't tell you where with the information at hand but I would start with the photocell you just replaced.

If you check your circuit and it is below the amperage rating of the circuit breaker. Then you should take a good look at your breaker. You will need an infrared thermometer to check the temperature of the breaker. Turn it off let it cool down for and hour or so and then turn it back on and see if you are getting a temperature rise on the breaker. You can also check this by hand by just touching the breaker but make sure you don't burn your hand. Yes they can get that hot. Some breakers over time and tripping can weaken and this type of breaker is thermodynamic so if the bimetal part is weak it will heat up and trip without overload. If this is happening I would just replace the breaker and inspect it for loose or corrosion connections at both the connection of the conductor and the connection to the buss. If necessary clean and make sure your connections are getting a good electrical and mechanical connection.

Safety note: If you have removed the panel cover to inspect the breakers make sure the power to the panel is turned off to avoid electrical shock. Or have a professional do this for you.

Good luck.