It will be safe to drive on for a while, although of course the sooner you can get it fixed the better. I had a car on which a rear wheel bearing went out, and I drove on it for a couple months where every once in a while it would make that scraping grinding sound for several seconds. Of course, it kept getting more and more frequent and long-lasting until eventually what happened was the friction from that rough grinding (the sound you're hearing) heated up the bearing so hot that it "welded" itself together and that rear wheel locked up.
So you're risking something like that happening every time you drive with a bad bearing, which is why you should fix it as soon as you can.
It will be expensive, but worth it!
