They are only in heat (breedable if you want to have babies) for a day or so...that would be the 6 hours to 3 days part, this is when they ovulate. Generally they will come back into heat (ovulate, accept the buck, etc.) after 21 days- an average heat cycle if they were not bred the first time around.
How do you know she was in heat? Tail flagging? Vocalizing? If you don't have a buck it's really hard to tell if she's in heat. If you have a buck they will try to get close to him...and of course he'll oblige. If you are sure she's been in heat for the last 6 days, she has a problem...probably cystic.
The best way to get her bred is to put a buck with her for 45 days or so that covers two heat cycles (avg- 21 days). Most likely he'll breed her the first time. When you see this mark the calendar 21 days later and make sure he's not trying to breed her again at that time. If she's bred she won't come into heat and the buck will not be tempted to "do his thing"