2.25" usually. I'm trying to remember, though, what the quilt in my avatar required. I want to say it was 3.25"; it finished with a 1/2" wide binding on both sides. Supposedly the wider binding is "better" for baby quilts. I'm not entirely sure why ... maybe because it lays flatter???
I know some people add 1/4" to their unfinished binding width when they machine stitch it; perhaps that's how they got to 3.5". Or else lynnie could be right & it was just a typo. Sometimes I will audition a short section of binding to ensure I like how it looks. Perhaps you could try basting on a small section of 8-12" with the wider binding to see how you like it.