The license is valid (or rather the type rating is valid) for 2 years after your PPL pass.
So to keep your SEP valid you need to do 12 hours in the second year of your license. In other words you can do nothing in your 1st year if you want and your license remains valid. The second year you must fly at least 12 hours (for experience revalidation). One complete hour must be with an instructor. I think 6 hours need to be solo.
You can also revalidate by taking a flight test, instead of the 12 hours, which I believe is an abbreviated version of the PPL flight test (i.e. only the upper air work). I've not gone down that route so stand to be corrected.
I'm sure your local friendly flying school can tell you all this!
As BEagle points out this is all in the manuals and LASORS, but I'm off on holiday today, so I'm in a helpful mood!
-DB