Couple of weeks I'd say. Time is a bit iffy on the show.
Remember when Chandler and Joey wanted to be like Richard so Joey started smoking cigars and Chandler grew a moustache. When he walks in, Ross and Monica find it funny... meaning they what, hadn't seen him for a whole week or something? How do you surprise people with a moustache?
Depends on the person. I can grow one of the size Chandler had in like... maybe 4 days. Certainly no more than 5 would be necessary for me. His mustache wasn't that impressive really lol. And while the friends were around each other a lot, i'm sure they didn't see each other literally every day.
The whole point of the show was that they saw each other EVERY single day. You could maybe make an argument for Phoebe not seeing him for a few days but not the others and especially not Monica.
The point is that they saw each other all the time, but all the time doesn't necessarily mean LITERALLY every day. Each of them had their own lives, they had jobs, relationships with other people, sometimes they went out of town. Sometimes they might be too busy to see each other. They lived in close proximity and saw each other a lot but it wouldn't necessarily be all six of them every single day, except for the ones who were roommates (and even then not necessarily because they might spend the night elsewhere sometimes, like at a girlfriend/boyfriend's place). I live in the same house with my brother and I don't see him every day.
In what instance would Chandler and Monica not see each other for an entire week? They hung out at Monica's almost exclusively and constantly borrowed food from her. The whole point of the plot was that they enjoyed hanging out with Richard thus inspiring Chandler to grow a moustache.
Are you suggesting that Chandler hung out with Richard for the whole week without ever seeing Monica?
I think Monica and Chandler probably saw each other daily given the fact that they shared a hallway with doors facing one another. Maybe they might have seen a day or two pass without bumping into each other but it seemed like their friendship relied heavily on Monica feeding the guys....so seems like they hung out daily.