Exactly the same as for a PPL.It is the aircraft rating that determines the number of hours. For a SEP Class rating you only need to do a test once every two years.
Not quite sure where the 7 years comes from. If you have taken ATPL level exams, you have 36 months in which to obtain a CPL and an IR, if you do not, then you will have to take them again.
The only 7 year reference is if you have a CPL/IR, then the longest you can go without renewing the IR is 7 years if you wish to retain your ATPL credit.