Ask HN: How many card expiration emails are acceptable?

Hi guys, first post - woo! So I'm toying with PayMill and their API's - so far, so good.

Here's my question: how many card expiration emails are acceptable in a month?

I'm thinking first day and last day of the credit card's expiration month. Obviously, I'd like to ensure subscription payment cards are up to date in order to continue to get payments rolling in, but I don't want to nag the user. Should I do one at 14 days in too?

What do you guys do/suggest? Cheers!