An offer of full and final settlement is put to creditors for their consideration, and if accepted then the IVA can conclude once all creditor claims have been agreed and other assets (such as PPI mis-selling claims) recovered.
There is not set period for IVA's to run over - they can be for as long as is needed. I once knew of a case which was to run over 30 years, and I myself have supervised one to run over 17 years - but for a full and final settlement, where HMRC are not a creditor, it is usual for them to conclude in the first year. If HMRC are a creditor, then I usually provide for a further year, as it can take them ages to submit their final claims.
Are you in an IVA already Diane, or just considering one at this moment in time?