Most women who try Clomid would be monitored each cycle to make sure it's working correctly and causing ovulation. The dose may need to be adjusted up or down to get the right number of follicles to mature at a time (preferably only 1 or 2). If this is happening and you still don't get pregnant on your own then you may go on to have Clomid at the start of the cycle and then an IUI. They usually recommend 3 IUI cycles before considering something else. If Clomid fails to make you ovulate even with adjustments to the dose each cycle then you may have to go onto injections.
The length of time you should be on the Clomid is really dependent on the progress you make each cycle. Some people get the right dose on the first cycle while others may take months to get to that point. Then your RE may want you to try a number of natural cycles before going onto IUI so it's really hard to say.
I hope that has been some help to you.