Somewhere between day 45 and 48 for me, I don't remember the exact number.
My friends and I don't have much time to play, so on one hour days we only get maybe 2-3 two hour (and therefore two in-game day) sessions per week. So that's about five game days per week we can get through.
That means that to play for (for example) 45 days we'd need to be playing the same game for three months without a reset. Given how often someone's computer will need to be reset or there will be a new experimental or something like that, we've never had one continuous game longer that that.
Our current game is only on day 4, since a power cut while playing corrupted our old game's save file last week and we had to restart once more.