Depending on their breed and size dogs usually begin to "heat up" or "season" after they are six to 24 months old. Between the ages of 6 and 8 months bulldogs usually start going into heat. The female French Bulldog reaches sexual maturity between 6 and 9 months of age at which point she will go through her first heat cycle.
Then some may begin to cycle every six months if left un-spayed while others may take 1 to 2 years to develop a regular cycle and may cycle irregularly until then.