Porting time limits

It used to be the case that some networks would only permit a port at the
time of initial connection and some retailers didn't handle ports into
certain networks and/or charged extra for that.

It now appears that Voda, Orange, O2 and T-Mobile permit ports after
connection (with VM and 3 possibly /only/ after connection) but is there a
time limit? Can one port in /anytime/ after connection or must it be
within, say, the first fortnight/month/whatever?

I'm looking at T-Mobile in particular at the moment but a more general
answer would also be appreciated. Network website FAQs make no mention
that I can find but that doesn't mean that there isn't something hiding
deep in the TnCs. Answers, links or clues all appreciated.

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