Hard to comment without knowing you start weight but as Moonbeam says, it is an average of a stone a month. I have gone down a clothes size for about 1 and a quarter stone. So, If you want to go down 3 and a half clothes sizes I reakon that would be about 4 and a bit stone? So, say about 4 and a half months, taking you to mid October so plenty of time before January.
This information is based on my size and what I lose and everyone is different, but I hope it gives you some idea!