Generally speaking, how much time needs to go by before it would be natural to use 久しぶり?
Is 2 weeks too short? A month? Where is the line?
Does frequency affect the usage of 久しぶり? As in, you meet regularly every 2 months, so you wouldn't say 久しぶり?
1) You regularly go to a gym and get to know several people there, but because you sometimes skip days as do they, after seeing them 3 times a week for a few months, a month goes by without seeing them. The next time you meet, would you say 久しぶり?
2) You only rarely go to the library, say once every 3 or 4 months, and someone you know works there. That is the only place you see them. Do you say 久しぶり every time?