I've heard anywhere from 8-12, depending on how much gas you use cleaning and stuff.
My local ind. gas shop said they normally do swaps, but since 10lb wasn't a popular size they were happy to refill it.
YOu might want to call a few places. Some places will only do an exchange, and some will fill, and finally some will fill , but you have to leave them there. If this is a nice new aluminum tank, you might want to find one that fills them. And just so you know, they usually charge about the same for 5, 10 or 20# fills. So if you ever get another tank, get a 20# tank, its cheaper to fill.
My local ind. gas shop said they normally do swaps, but since 10lb wasn't a popular size they were happy to refill it.