Everyone else beat me to it. Estrus in a doe lasts 36-48 hours. If she does not conceive during that timeframe, she will cycle back into estrus in 28 days. She will continue to do this for 4 or 5 cycles, if she is healthy enough. I emphasized conception above because a doe can get bred and not conceive.
Some does are barren (cannot conceive for a number of reasons, just as some human women can never conceive). Research on the percent of does that are barren varies. I've seen stats as low as 0.5% and as high as nearly 10%. This is important, as some does that are barren still go into estrus. Because they cannot conceive, they will keep cycling back into estrus every 28 days. This is one of the causes of hunters seeing chasing in March - a barren doe coming back into estrus on her 4th or 5th cycle. In addition to barren does, as others have mentioned, in a health deer population, a given percentage of female fawns will reach the necessary body weight to enter estrus, and they usually do so in January or February. In TN, I've seen upwards of 25% of female fawns able to reach estrus their first year (as a fawn). In the far north, I've seen 80-90% of female fawns achieve estrus as a fawn.