If a bracket comes off in the first few months of treatment, while so-called ‘soft’ archwires are being used, rebonding it usually doesn't affect how long the treatment takes.
At more advanced stages, when a ‘stiff’ or finishing archwire is already in place, a bracket coming off may mean going back to an earlier archwire size so that the displaced tooth can be brought back into line. In some cases this can extend treatment by 2–3 months.
So, on the whole, it is important to avoid brackets coming off, to prevent delays and extra corrections.
This answer is for general information. Your own case needs an examination by an orthodontist.