How often do you maintain your relationship with past acquaintances friends etc ?

I know a lot of people but I find it really difficult to maintain contact with most of them. I can manage a few in my close circle and some acquaintances but definitely not more than 20 people.

It doesn't take much time for the gap to widen and before you know it they'll start treating you like a stranger.

So I was thinking whether there's something like the for relationships.

Maybe finding out the optimal time interval can help me set reminders to connect with a few people now and then.

Well this was just a though that randomly came to me. I thought I'd ask the wonderful people over here.