How many PSA keep in the case of FULL update?

Hi Guru,

I am wondering how many PSA do I have to keep in the case of FULL update?

1 - Concerning the Master Data process chains in FULL mode is it necessary to keep the old PSA?

2 - Concerning the Transactional Data pc in FULL mode (with or w/o selection criteria) is it necessary to keep the old PSA too?

Usually for the delta pc I keep 45 days of PSA.

Is there a best practices document?

Samuel