The book I have recommends every second day. They also say that your 'fertile period' for a 28 day cycle is from day 6 to 17 (they say you O 12-16 days BEFORE your next AF and that sperm COULD last up to 6 days). I try to DTD every second day during this time. It worked the first month we tried. The second month I think I O'd later than expected to we missed the eggie...