The remote controller will start a sort of slow beeping when the batteries are low (almost impossible to miss)
It will still run normally for a while even then - no need to rush home and land - though it would probably be unwise to fly far if it starts as you take off.
You should get several weeks of use before that happens, I think mine did about 40 flights at least on the supplied AA batteries.
It makes sense to have a replacement set on hand.
The range extender is good for several flights but seems to lose charge even if not used - advisable to top it up daily if you are using it.