Roland Perry wrote:

Or they may pass a new law saying that all credit expires a year after
it's added - I think the New York equivalent has that rule.

Not quite.

Each physical MetroCard has a preprinted expiration date. When you buy
a new MetroCard, the expiration date is usually a bit over a year away.

The card will no longer work after that date, regardless of whether
there's any balance left on it. However, the remaining balance can be
transferred to a new card at any station booth or MetroCard Vending
Machine until two years after the expiration date (recently extended
from one). After that, I believe the balance is lost permanently.

(If the card is an unlimited card, in the middle of its period of
validity, when it expires, then the station agent can't do anything
about it. I'm pretty sure it can be mailed in for a prorated refund or
for a new card containing the remainder of the unlimited period.)

If you're an infrequent visitor to New York, I suppose that means that
you should try to use up your balance before you leave New York.
Otherwise, it's largely not an issue. There is no charge for the
MetroCard itself, and unlimited cards are always dispensed on new cards.
--
David of Broadway
New York, NY, USA