Timing of SA

When does a separation agreement need to be signed, compared with the time a couple actually physically separates? Can it be afterwards? If so, how long (days? weeks?). And is there a limit on how soon before the couple physically separates then can sign (and have notarized) a SA?

If an agreement is signed when separation is impending and it actually does happen the Agreement is valid. ( I would say impending would be no more than 2 weeks later). An agreement can be signed at any time after the parties separate.