Roleplay threads don't have a set expiration. The originator can reopen a thread weeks, months, or years later without any problem. Some RPs are really slow and take take months between "turns". On the other hand, random nations posting for the first time in a thread that's been idle for just a few weeks can be considered gravedigging.
Once the OP asks for the thread to be locked, you can consider it "dead". Of course, they can ask for it to be unlocked later if they wish. The mods would rather they just leave them alone until/unless they feel like picking back up.