@WolfOfEve: The simple answer is you can't. That's the RPers responsibility. You can ask them not to join if they can't handle it, but that still leaves it in their hands with no way of verifying. Random RPers earnestly can't be trusted to make their RPs an obligation. If you try, you will wind up disappointed. Even good RPers have a tendency to just... fail to predict exactly what they can handle. It's kind of hard.
If people are not posting, you should contact the people who are not posting directly and find out what is going on. (And I don't mean via an OOC thread. That is not direct. Use VMs or PMs) If they aren't going to post, find out who is willing and make it work with who you have left.
An RP cannot start with the same members and end with the same members... unless... it's really dang short. You need to be ready to move on without or even replace members who simply aren't there anymore in order to preserve the life of a roleplay.
Perhaps most importantly, under no condition should you wait weeks to do this. If you wait weeks to deal with inactivity you've made the problem worse in the process and may have, in fact, killed your own roleplay for no good reasons. You are not nagging your RPers when you ask them if there's something wrong keeping them from the RP. That's just standard communication.
Remember. RPing is just a hobby. If someone signs up for a school sports team you own, and they just don't show up (And don't talk to you about it and reach an agreement otherwise). Are you going to sit there and wait for them? Go one man down? I don't think so. Same principle. An RPer has no more right to a spot in an RP they aren't participating in than an athlete does to a sports team they aren't participating in.
It's up to you to decide exactly how long to wait before asking someone. The only correct answer is "Not too long". Obviously, being too harsh is also not healthy for an RP. I personally follow a 3-day rule for personal situations, and a one week rule for impersonal situations. (Meaning, if I'm not directly RPing with someone, I'm less strict, because the people they're RPing might not care as much as I might.)