What's worked for many people at other banks (though I don't know specifically about Chase) is to either call or (if available) secure message the bank after you have activated the card, and ask "how much do I have to spend to get the signup bonus and by what date". If you ask that way, you both verify that the bonus is attached to your account (at banks, like Citi or Amex, where you can approved for a card but decline for the signup bonus) and you get the actual date if you need it.

At Citi it tends to actually be 3 months + 14 days from approval date (probably because of a famous newspaper column complaint a few years ago about someone getting denied for the bonus because they completed the spend a few days too late because they miscalculated it). But I haven't heard of any other bank consistently "padding" by anywhere near that much. (And, again, I don't know what it is specifically for Chase.)

And, in any case, what "3 months" has meant up to now at a particular bank could change without warning at any time, so that's why I think the best thing is to verify what the end date is on your offer by asking after you get the card in your hand, and not just go by some chart (if it exists) about what each bank tends to mean by "3 months".