I have created a workaround:
if month(getdate()) = '9'
delete FROM dbo.CounterData WHERE CounterDateTime like '%-07-%'
if month(getdate()) = '10'
delete FROM dbo.CounterData WHERE CounterDateTime like '%-08-%'
Running this statement for example on 1st day of September, will delete all data from july (month 07). It´s not the best way to do it, but it works. Assuming this runs every month, you guarantee that only one month of data will be available
I put this code (validating 12 months) in a SQL Agent Job running on 1st day of each month.
For now its what i need
Thanks a lot for all sugestions