Any idea how long after purchase one can wait to activate?
I get $45 cost on the above link.
$28.50 for 3 months unlimited + 15GB a month on Simple Mobile---be aware of the shipping conditions
The standard Mint policy is 45 days. I've pushed it beyond that by a few days once.
I had bought at $45 ea 2 weeks ago. I called in to get price adjustment last night. Should see $30 back (2 SIMs) next week. 
The Mint 45 day limit is BS. How would they know when you bought it? If you purchase it on eBay, you're purchasing it from somebody else. When would the 45 day countdown begin? When you buy or when they bought it. Go by the 'activate by' date on the card. I've held mint sims for over 90 day before activating them twice now without issue.
In theory, the clock starts when the item is scanned at original point of sale. If you buy it on Ebay from someone who bought it and is reselling, the clock is already ticking.
The 'activate by' date on the card is T-Mobile's expiration date for the electronic SIM card itself, after which T-Mobile may expunge the SIM card from it's database-- the 45 days is Mint's requirement to activate the plan you bought. Agreed that it adds confusion to have the SIM expiration date on the card.
I've waited a little over 45 days. Glad to hear you're been able to wait over 90! The risk is if it doesn't work, you will have no recourse with Mint past the 45 days. Mint CS may, or may not, be inclined to assist.
Guess it's all YMMV. When you buy from Best Buy, it's scanned and activated. Perhaps sellers on Amazon and eBay buy them wholesale and that has longer time for activation.
I got both of mine from Mint's superbowl promo. Actually my neighbor go one for me and I got the other. I activated one as soon as I received it and the other 92 days later......
You are a risk taker aren't you? 
Did you receive the following email from Mint? I got it on Mar 13th. :
"Activate your free plan by March 19
So, remember when you got one of our free 3 month plans on the day of the big game? Well, you haven’t activated it yet, and the SIM card that came in it actually expires once you reach the 45-day mark…it’s a real thing.
We want you to advantage of all that free wireless you got, so all you need to do is activate your plan asap. Oh, and while we are cleaning up expired things, it couldn’t hurt to check how long that jar of mayo has been in the fridge. Just saying."
I took them at their word and activated on (what I thought) the last possible day.
The timing with the free refills worked out in the end anyway.
I'll be attempting one of the superbowl promo SIMs in June. We'll see how it goes!
I guess if you're going to take a risk on activating a MINT SIM, the superbowl promo is the best choice to take the risk on.
I didn't get that email since I activated mine right away but I assume my neighbor did. His was the one I waited on. Not really much of a risk considering that it was free.