There is no "right" number of days. Typically, it seems to work best when the range is from one to three days per week on average. In some cases it might be higher, but there are potential problems when you get to four or five days per week - it's harder to continue feeling that you're really part of the work group, it's more difficult to schedule meetings that you must attend, and the logistics of getting work to and from the office can get complicated.