They are valid for 5yrs (RSSB), however with some operators like FGW or SWT, its 3 yrs.
For instance;
STAGE 1
- Assessment taken; 01/11/13, EXPIRES with said TOC; 01/11/16. True RSSB expires; 01/11/18
STAGE 2
- Assessment taken; 01/03/14, EXPIRES (TOC); 01/03/17. RSSB; 01/03/19
In theory should your stage 1 lapse and stage 2 is valid, then you would only need to re do stage 1 (providing you got to DM/training before the expiry time) ...that said TOC can still apply the tests even if your still in valid.