If you have the money to get more gasoline you should always fill the gas tank back up when the gas gauge reaches 1/2 of a tank.
That's what I do is once the gas gauge reaches 1/2 tank I fill it back up and sometimes I'll top off the gas tank and put it back to full even after the gauge moves just slightly away from full.
I like to keep my gas tank full so I never worry about running out of gasoline.
There's always the chance that there could be a natural disaster or other emergency and I need gasoline to either evacuate from the area or leave to see family etc.
So I always keep my tank full most times and go to the gas station and fill the tank back up when it reaches 1/2 of a tank or sometimes sooner.
If you can't do that then you can fill up once the gauge is nearing empty but it's best to not wait that long.