AllInGoodTime - I'm afraid there is no simple answer. It very much depends on what aircraft type you are assigned as a S/O (and there is no choice given).
B744 S/O's tend to get longer trips, whilst A330 and 747 (Classic)S/O probably tend towards shoter trips. That said, most S/O flying is quite efficient, resulting in approx 30 days off (or more sometimes) per 56 day roster.
First F/O slot is likely to be on a 737 or 767 (though who knows where we'll be by the time that comes around), with less efficient domestic and Tasman flying. Time off is then more likely to be of the order of 12 or 13 days off per 28 day roster.
Hope that helps.
Esrev.