Perhaps every 6-8 months....my bitch goes into heat closer to every 8 months. The amount of blood drops are minimal in her case....she usually cleans herself...licking herself.
"A female dog is diestrous and goes into heat typically twice every year, although some breeds typically have one or three cycles a year. The proestrus is relatively long at 5–7 days (9 days), while the estrus may last 4–13 days, with a diestrus of 60 days followed by about 90–150 days of anestrus. Female dogs bleed during estrus, which usually lasts from 7–13 days, depending on the size and maturity of the dog. Ovulation occurs 24–48 hours after the luteinizing hormone peak, which is somewhere around the fourth day of estrus; therefore, this is the best time to begin breeding."
The previous two females I had which were spayed never had any bleeding.
My current bitch I do not put diapers on or anything similar. However, I do pay attention to any blood drops that she doesn't clean up herself....perhaps I clean up 4-8 little drops a day.
My current shep's estrous cycle is a non-issue basically EXCEPT making certain she does not get impregnated via the gentleman callers from around the neighborhood.