Tub spouts come in two general varieties: press-on, and screw-on. Each of them needs the pipe or fitting to be a fairly specific length beyond the finished wall. When the pipe was installed, the wall probably didn't have the finished surface, so it is risky cutting the pipe off. So, assuming you've got a spout that needs to be screwed on, you'd either attach a threaded coupling onto the stub by solder, or tear into the wall and replace with a drop ear el, then install a brass nipple, or get a push-on spout. If you paid the plumber for a finished install, he should come back to finish up and can do what is necessary. If you paid only for a rough-in and didn't specify the type of spout, he prepped it for a push-on one. Should still work for a screw on one, though. You need to measure carefully as you'll almost certainly have to cut the stub off if you want the length from the wall to be exact once the fitting is soldered on.