For anyone interested in the answer: about 11 days.
I suffered a small skin break on day 26 out of 33 and it was dressed with mepitex until Radiotherapy finished. My skin continued to break and blister until abut 10 days after despite using Flamazine and mepitex dressings daily. At that stage my underarm was completely bright red and was weeping all over. The area was about 6 inches square.
By day 11 I noticed a few tiny pink spots of new skin and now day 14 after finishing there are only 2 areas still weeping (about a sq inch each) so its is slowly improving.
However, my back which did not get a direct beam has now started to break up but not as bad as the underarm. At least I know it should be fairly healed before starting chemo in 2 weeks.