I think once you become friends (whatever your definition is) then time between talking doesn't matter much, I agree with the "pick up where you left off" being a good indicator. I've had a number of friends met through MTB where this has applied.
One close ("dear") friend that I've known since school communicates with me every now and then, we haven't chatted directly in a few months, we have a chat group with both our partners included, that we talk on infrequently, but I would know within hours if he died.