I voted never because I have only shaved twice in one day, a couple times, so in all honesty the answer is functionally never.
I shave at night, so going out is never an issue. However, sometimes on rare occasion my shave is so bad I need to touch up later (or in the morning) so on a couple occasions I have shaved twice in a day.
It is not often, but some afternoons, say after a run and a hot shower, I feel like shaving again, just for the fun of it. I never try for BBS, and I use the mildest razor known to man (the much-maligned Feather AS-D1), so I always have a bit of stubble later in the day.
I have to be careful, though. I don't want to shave so close in the afternoon that I can't shave again the next morning!