How long does I can of pop last? As long a it takes you to drink it.
It is the same with your battery. It's life depends on your use.
Our TT with it's original battery will last us two or three days if, we turn off the antenna preamp, switch the frig and water heater to propane after we changed to LED lights. We know the voltage is low when the water pump will not build enough pressure to shut itself off. The water pump is our biggest current draw.
Left in the yard the phantom loads will drain our battery in about a week or so. When we expect to Boonedock for a few days we carry an extra battery in and old picnic cooler. I have managed to stay an extra day or two by charging the battery with a set of jumper cable of the truck. It is kind of a crude approach but work if necessary.
If you intend to Boonedock you gear up for it.
We have a 25ft. TT with no special inverters etc. for 120 volt operations.
Good luck