As far as I am aware it should finish at the 6 year anniversary and will only finish early if you can offer a full and final settlement.
An IVA can last longer than six years - depends on the circumstances.
Sharing from experiences of dealing with debt
The greatness of a man is not in how much wealth he acquires, but in his integrity and his ability to affect those around him positively.
Bob Marley. http://kallis3.blogs.iva.co.uk
An IVA can be arranged for any period of time, by agreement. It can be shortened, once set up, by full payment of original debt, plus fees and possible statutory interest, of by creditors agreeing a full and final offer.
Your 6 year term --- was it 6 ( as opposed to the usual 5) because of the rental property ? If so it shouldn't be extended. If it was 6 for other reasons, then it could be extended by another year in lieu of equity release.
IVA's can also be extended to make up arrears, payment breaks or to offset a reduction in regular payments during the term, by agreement.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014