I just got mine. They have a terms & conditions page in the e-mail (which sent me to an invalid page, well done promotions/marketing team), but I found the actual page by googling.
https://www.playstation.com/en-gb/legal/march-cashback/
I'm currently searching through it to see if it actually states the redeemed content/funds will be revoked or removed after a period of time, but not seeing anything.
Though to be on the safe side, the e-mail did state as a closing line as others have pointed out, so I guess I'll find something to spend it on in April:
Go back a few pages in this thread, it was discussed and the answer was that
you only need to use the code, the money is counted as real money, meaning doesnt expire