friendship term limits

When do you stop being friends with someone?
We're not talking if you are slighted in some seriously destructive way, or realize that the person you thought you knew wasn't quite that person.  We're talking . . . term limits.  Do they exist in friendships?
It's best not to put constraints on friendships.  They are living, breathing, albeit invisible, bonds between two people. As such, where do they end?
Or do they?
I have had "friendships" where it seemed like I was doing all the work--so I stopped.  And now, YEARS later (what, almost 20 now?) I haven't heard a thing.
(That's sad for someone . . . or perhaps everyone in the scenario).
If you have an intense friendship (daily communication) for any length of time (say, several years) and then it stops--or trickles, say, to once a year, or every other year, does that still make you "friends?"  Do you lose that intimate connection?  Are you strangers passing in the night, like an insomniac and sweet, sweet dream time?
Perhaps friendships aren't very different than relationships based on love . . . they take work to cultivate and prosper, and if they're ignored they're just dying on the vine.  (Wait, this isn't facebook again, right?)
Perhaps friendships can downshift into aquaintanceship.  Not strangers, really, because you have that history. But it has lapsed, and while you can slip back into the familiarity the bond is lessened, weakened, tattered and torn.  But on some level, the friendship still remains.
Yeah, that sounds positive.  That's what it is, right?  
See you around, friend.