When does a cavity get bad enough to the point that you need to get a root canal?

How long would that be?
Update: wow thanks for the responses, especially to sarge and heather. The reason why i'm asking this question is because at my most recent teeth cleaning(about two weeks ago), they discovered a cavity in one of molar teeth and they said it was already in the enamel. The problem is that i have braces and my ortho... show more wow thanks for the responses, especially to sarge and heather. The reason why i'm asking this question is because at my most recent teeth cleaning(about two weeks ago), they discovered a cavity in one of molar teeth and they said it was already in the enamel. The problem is that i have braces and my ortho said he didn't want to disrupt his treatment so he said to have the dentist treat the cavity at my next ortho appointment which is the 30th of this month. I'm just worried that its too long of a time to let my cavity sit there and progress its damage. I'm really not looking forward to a root canal at all.
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