A 6 month administration period is the usual term built into an IVA under usual Ts&Cs, however, it usually happens a lot faster. Some can manage it in weeks, most a couple of months. Unfortunately the delays are not always under the control of the IP and, often, non-response from creditors holds things up. Admittedly, I think, IP's are often too gentle with their dealings with creditors and give them more time to respond than they are officially entitled to. As you can guess, a lot also depends upon how pro-active the IP is approaching the final payment time (not as though it comes as a surprise ! ).
There is, in all honesty, little you can do other than pester (gently) -- squeaky wheel and oil and all that.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014