My understanding is that saving a draft will keep it for 24 hours. So, if you start a thread and save it, then go back to that forum & hit "new thread" your draft should show up in text in gray. I just tried it and it worked for me.
BUT, if you aren't sure when you're going to come back, you might want to write your text in an email & send it to yourself or write it in a document and save it, then post when you're ready.