In real life a lot will depend on the charge remaining in the battery at the time you discover the alternator is not charging it.
In gliders, we can fly all day on a single battery charge powering the radio, vario and computer, but there are those with a two or three battery setup.
By switching off the master, you also turn off the turn co-ordinator gyro which takes quite a bit of juice.
If you are on a flight plan at night, tower will make sure the lights are on at your expected arrival time. Of course with pilot controlled lighting you are on your own and a handheld would come in handy.