IB50 form - waiting time for result/medical invite

First time on here though a 2nd year member- please can anyone tell me if there is a backlog with dealing with IB50 forms? I returned mine to Atos on 1st December, to Nottingham office (for Derby area) which Jobcentre say was received by Atos OK. They made no further comment when I rang them. (It was the second IB50 form sent to me within a year - I had only filled one in in May - even though my health problems (several including osteoarthritis in multiple places with spinal degenerative changes) are non-reversible and chronic and I have problems with some painkillers, anti-inflams etc.,so pain not well controlled - you get the drift. It was a nightmare to fill in but your site helped a lot.
Well I am supposed to change over to state pension in early July this year, so was assuming they would not change my benefit to ESA, but do you think they are hanging on to my form till Feb/March so they can then ask me to fill in an ESA50 instead?
What is the usual waiting time after returning the IB50 form to hear if you are successful in continuing on Incap Ben, or indeed whether you are required to go to a medical? What's the 'record' on longest wait? Many thanks.
PS I am having procedure on one hand in hospital this week so may not reply to comments immediately but any advice very welcome. Thank yo.

You won't be changed over to ESA because you will be retiring in July.

Thank you, I thought so, but was not sure if this gov had moved goalposts again! Just my IB and DLA LRC to worry about then!

Just to say I have now just heard from DWP and my Incap Ben will continue until my 'normal review date' of late July 2011 - relief - (but I will change over to state pension early July, so no more reviews to worry about, at least for this, though LRC of DLA may be under threat soon..) thank you so much for the helpful advice on your site. I will continue membership after retirement to support others and also as my only grandchild is on DLA HRC awarded for two years (No mob comp awarded strangely enough...) He is in hospital again this week bless him. I think your site is great and has also opened my eyes to how many of us are being treated, I now follow Broken of Britain and Black Triangle on facebook.Thank you again.