It's not a limit, just from my personal exposure in the overall drone delivery, 3-4hrs would provide enough time with margin for any safe delivery, beyond that is definitely better but probably is too hard to achieve. Keep in mind the actual flight time will be less than that, accounting for payload weight and environment like wind and temperature. In cold weather like Canada, the batteries will consume some of their energy to heat themselves before taking off.

What's the math that gets to 3-4 hours? Do the drones usually multiple deliveries in a flight? In my metro I would have assumes that any place is less than 30 minutes flight time from a Whole Foods or Walmart for example. How long does the actual safe delivery take? I assume it just lowers the goods and doesn't wait for the customer, right?

Edit: The Internet tells me that delivery drones fly at 40-60mp/h which is much faster than I assumed and makes the 3-4 hour window even more surprising to me.