Hi, the reason that your SS only lasts for 12 weeks is not diet-related, it's to do with your health.
There have been many medical studies done on VLCD and their potential side-effects and the outcome is that the NICE guidelines (a kind of intruction book for health professionals) say that even if you are being clinically monitored you shouldn't stay on a VLCD for more than 12 weeks as it can be dangerous for you.
This doesn't mean you have to come off your diet altogether, just that for a week or so you will need to add some additional calories. You can then go back to SS for up to another 12 weeks.
I'm sure KD or your own CDC will be able to explain how much and what sort of food you will need to eat and for how long. Because it's only for a short while to give your body a bit of a break most people don't seem to notice much difference in their weightloss.