I can speak as someone who's been on both sides of this particular situation. I've got some roleplays I'm holding onto, but I've also put my writing partners through some long delays.
Generally, I don't tend to replace anyone; instead I start a game/story over with a new partner, but then I primarily do 1:1 freeform, not multi-player system games. In terms of my 1:1 roleplays, I usually keep an eye on log-in information. If somebody's not logged in for more than a month, I will usually consider that roleplay to be inactive. If somebody's logging in and active on other things, hasn't posted in A&A about difficulties staying caught up with life and hasn't sent me a PM or anything, I'll send them a PM after a month or so. I make exceptions for people I know well. Those just sit until the person has a chance to post.
If the situation would involve a multi-player system game I was GM'ing (not that that's ever likely to happen), I'd want contact every week or it would be stated in advance that I'd reserve the right to replace someone. If it were somebody disappearing from the site, I'd hold the character in the background or make that character a temporary NPC. If it were somebody posting elsewhere and not making contact with the game or keeping news going in A&A, they'd be getting a polite 'poke' after ten days or so of inactivity. That's just about courtesy to the other players.
I do keep track of all my roleplays, though. If you take a look at my Elluiki page, I've grouped my roleplays into 'active', 'on hiatus' and 'completed or abandoned.' That's kind of how I look at them. 'Active' is pretty self-explanatory, but in my case it means I really do want to keep up with that roleplay, and I'll be giving it my best shot. Like you, Sherona, I have several stories with TheWolf and a few with others where the stories are so great I don't want to give them up with that particular person, so I'm holding them for if/when I or the person I was posting with reappears or wants to start up again. Those are in 'on hiatus'. The entries under 'completed or abandoned' are those where the story/game really is over, where my writing partner is no longer on the site, or where I do not wish to continue with a roleplay. Anybody can tell at a glance which games have what status with me.
So... based on that as background knowledge of my preferences, I'd suggest that you contact the person involved and ask what's up. I wouldn't have let that wait even 60 days. The time you've given it now is plenty to be politely patient. If there's no response in a few days and the person continues to log in, I'd consider that lack of reply to be the answer, and either get a new player/writing partner, toss that concept to the back burner for a while, or start the concept fresh with someone new.
If somebody would come back to one of my 'on hiatus' roleplays, I'll pick it back up again if I have the time. If somebody would come back to one of my 'completed or abandoned' roleplays, I'd let them know I'm not going to be continuing, but my Elluiki page actually does that for me. If it were related to a game where I'd been NPCing the character, I'd have to think it over about letting the person come back. If the character was dead, I'd probably say a polite 'not at this time,' and if there was somebody new playing the character, I'd politely tell the person who disappeared that this was the case, but then I'd never give a character to a new player unless it was a player who was logging in and ignoring communications. I'd not want to hand a character over to someone new if a player was away ill or dealing with an emergency.
Rather than an official Elliquiy policy, I think it's something that should be on the ONs and OFFs of anybody to whom that matters, or one could always make stipulations in the advertisement for or organizational phase of a story or game. There's nothing wrong with negotiating something up front about participation, but I don't know that it would be right to set site rules for that, since experiences and needs vary on here so widely. FWIW, any time I've run a multi-player game, I've had rules for participation that include the time at which a character goes to NPC status and when they are removed.
I hope that makes sense, even though it was a long-winded way to say that I agree with GT and Sherona.