I went to a dentist who said that with my teeth, I'd probably only need them for about a year and a half. The current dentist I am with has decided on 2 years and I am on a payment plan for that amount of time. If by chance I need to wear braces longer, do they charge you extra? I'm just curious.
Total fees are usually based on the complexity of the case, which does tie in loosely to the predicted duration. However, it is most common for people to be quoted a flat rate fee for their treatment, and that is the fee that is applied whether the treatment ends up taking longer, or whether they are able to get out of their braces sooner. The monthly fees people pay are usually just that flat rate, less some downpayment, divided between the approximate duration of treatment.
But it all really depends on your contract.
Some gotchas that I have seen include needing to pay the remaining balance in full before getting the braces removed when treatment ended earlier than expected, and before all installments had been paid; and in one case I heard of someone who was told the flat fee only applied if she had her complete treatment at that provider, and that if she transferred, her contract instead indicated that she would pay itemised billing - records, hardware, and a per visit fee - this ended up being more than the flat fee, so she was pretty much locked into staying at that provider.
So, check the contract.
Where I work, the amount charged is the amount you pay, whether your treatment time is exactly as suggested, less or more.
The contract does allow for additional charges if treatment time is extended due to patient non compliance, but I've been told that's not been used, but is recommended to be included in contracts.
Definitely check your contract, and also ask your current dentist, as this is the person who will be able to tell you what they expect should happen.
Good luck!
After 5 years, 11 months and two days of stainless steel brackets ... my teeth now have upper and lower bonded, gold wire, retainers and removable clear retainers!