At the end of June I paid for dinner in Mexico for my family on my Visa. The charge still hasn’t posted to my account. Understanding that vendors often will hold charges until the exchange rate is best for them, how long can they wait before they submit the charge?
Well, the vendor will get paid in local currency regardless of the exchange rate, and that won’t change. If you were charged MX$1000.00, that’s exactly what he’ll get, even if their government goes crazy, devalues the peso, and makes the exchange rate on the order of 100:1. He’ll get his 1000 pesos, and the favorable (to you) exchange rate will make it post at about $10 at that rate. 
On a more realistic note, where was dinner? There’s a major portion of Mexico City that’s currently all but shutdown due to a political dispute. So the billing guy can’t go to work, can’t process the charges, and it won’t post until the Mexican Army clears out the streets later this month.
This was at a restaurant in the Tulum area.