It's dependent on the wire that you have running from the vehicle to the trailer, and the wire from the trailer to the battery.
If you have 4 gauge cable with an Anderson connection between the vehicle and trailer then it will charge as quickly as your main battery.
If you have 10 gauge wire (standard) running back from the vehicle charge system then it takes a lot longer.
The best thing for you to do is put a volt meter on the trailer battery to get some idea of your charge after driving for a certain length of time.
A word of caution here. If you are discharging your battery and then only partly recharging it from the vehicle on a regular basis the lead plates in the battery are going to become sulpherized. This will reduce your battery to around 2 years instead of the 5-6 year life.
To prevent this we recommend using a smart battery charger when the trailer is not being used.