Falling out happens pretty suddenly. One day they are good and the next one decides to start chasing the other, and it escalates from there. It can happen at any time without any indications, though I guess more likely around 3-6 months when hams go through their teenagery stage. I had to separate my first two robos after 5 months of their life together. My two younger girls who live with Leela are now between 3 and 4 months, and Viola started finding things about Olive that she doesn't like
It's a hit or miss with cohabitation. I think it's always worth giving hams a chance to live in a pair, because I've seen the beneficial effects it has on their state of mind, and if it doesn't work out--it doesn't work out; there are options afterwards.