A woman typically ovulates about midway through the
cycle. Therefore in a 28 day
cycle a woman will ovulate ~day 14. Since your
cycle is longer you will probably ovulate around day 17 or 18. To help detect when you ovulate you can try a number of techniques. Two relatively easy ways are to track your basal body temperature, or use an ovulation (or LH) detector kit. Once you have an idea of when you are ovulating you can then time intercourse accordingly.