I've heard that some places got something like "three days rule", which I guess is that if one hasn't post a reply(in the rping-part) or something like that... on the other side, I'm not totally sure how the rule works, but I think it may be a good idea. Because some are online pretty often(like me) like once or more a day, while other maybe is online not quite so often, like 1-2 times a week. By having this rule, I think it would be really good for both, and that one don't have to wait 3 months to get a reaction from the other one.
Sure, we don't have to have 3 days, maybe 2 or 4-5...
What do you think?

What happens to the person if they don't reply, though? Like the consequence? Is their character essentially erased from the rp? Or can people just carry on, and the character is just sort of left on the side lines?

I'm not sure, cuz I'm pretty new with online rp. But I guess the rest just keeps on going or something.
The other could always do something like: "... but before XXX could say anything more XXX fainted of exhaustion. 'Well', YYY thought, 'not wonder why when XXX is awake into the late of night.' YYY grabbed a plaid to place over XXX body, before YYY went to the room nearby..."
If someone doesn't know it here, I'm gonna search more excatly.