Assuming the UK court throws the maximum sentence for skipping bail he would be given 12 months in custodial sentence. But UK law says for all fix term sentences under 10 years you are automatically released at the half way point. Which means that practically he would only do 6 months in prison and then be released on license in the community under probation supervision. So considering he has already spent the last 7 years in self imposed imprisonment inside the embassy 6 months would be relatively easy. Although it would depend on which prison he went to and whether he choice to go into the main population or onto the vulnerable prisoner wings. I suspect he would be a target for a few beating on the main wings as he would no doubt piss some people off.
It will be interesting to see if he was correct all along and this was all about getting he extradited to the US or not.