am wondering how long it will take for someone to pick it up,
Anywhere from right away to never. My guess is that most books get picked up. But whether they get journaled or not is another question.
Only a smallish percentage of released books get journaled, and thus "caught" by our definition, but that's what makes this interesting.
If every book you released every where got caught every time there would be no mystery, no suspense, no intrigue to BookCrossing. And though I wish I would get more catches, as they are always fun, it is interesting to me to wonder which book, left at which place, at which time, will be the one I hear back from, and what the finder will think of it. It's serendipity really. Sometimes that journal entry can surprise you years and years later.
I release books hoping that someone will find them and enjoy them, the catches are the icing on the cake. I even release in spots that I'm unlikely to get a journal entry, like a retirement home where most of the oldies don't use the computer, or at a food pantry where most may not have easy access to one, because I know folks there will enjoy getting the books.
So try to enjoy the process of releasing books and not worry about whether you will hear back from this one. Set them free. It will make BookCrossing more relaxed and enjoyable. And of course, the more well-labeled books you release, the more likely you are to get a catch.
[And don't, in your labeling or online notes ask people to "register" the book, that will give the book a new number and you won't hear back from it again. Be sure to ask them to journal the book, not register. You've already registered it.]