I was originally going to say 2-3 days before I start becoming too caught up in my head in a negative way, and bored. But that's in everyday life if I am not working at anything or out and about doing things. So, if I'm stuck in my apartment, I'm usually ok for about 2 days - and that's without phone calls or any communication, and a lack of stimulation. Beyond 2 days and I become unhappy.
If I'm really stimulated- let's say I'm traveling somewhere new, or out of the country. From experience I know I can go 10 days virtually totally on my own, entertaining myself, without needing conversations outside of a few sentences here and there. But, in these situations, I am often surrounded by people or am actually DOING something externally - engaging in something in some way and not just totally in my head. At about 10 days, I hit a wall and crave a real connection. When I get past that day or two, I'm good once again and can go another 10 days without much of anything.
My IDEAL in 'normal life', which I strive for, is doing something or other either socially, with my boyfriend, or on my own but still out and about (i.e. a yoga class, or a hike by myself), three or four days a week, with a day to myself, in my apartment, in between all of those days.
But also, with close friends or my boyfriend, I could easily cohabitate with them and see them everyday, while still having my 'alone' time and I wouldn't get overwhelmed or anything.