The 48hours is once any non-refundable payment is made.
For instance: I'm going on
DCL in July. I've paid my deposit, and my room deposit for WDW after, but if I were to cancel right now I'd get my money back so in essence there's really nothing to insure.
But, I just booked my airfare. That is non-refundable, so now I must purchase the insurance within 48 hours if I want to make sure that the airfare is covered. I can also include the total cost of the cruise and the hotel in the insurance - I just have to make sure that my "amount insured" is high enough. So even though I haven't actually paid them yet since I've made my first non-refundable payment on the trip I might as well just lump everything in there. It's easier for the insurance company than going back and amending the policy every time you make a non-refundable payment.
Just an FYI for those folks booking airfare through Airmiles or Aeroplan - you can still purchase cancellation insurance for those flights. Just because they're "free" doesn't mean that you can't insure them.
(And this is why when I'm on a flight or in an airport and someone starts a conversation and asks me what I do for a living, instead of telling them that I'm a travel agent I tell them that I sell insurance! I learned this chapter far too well in travel school.)
ETA: Just to clarify - if you've booked something like a cruise then you certainly CAN take out the cancellation insurance as soon as you've paid the deposit, even though it is refundable. A person might do that to ensure that any possibly illnesses or medical conditions that might arise later would be covered. All medical conditions have to be stable for X period of time prior to the purchase of the insurance - let's say 90 days. Stable means no change in the condition, *no change in medication, even a decrease*.
Okay so for example Mary books a cruise for 2 in October05, sailing the following October06. Deposit is due October 05, final payment due August06. Mary doesn't have to take out cancellation insurance until August06 because before then she can get all her money back. But let's say Mary's husband Bob is diagnosed with a heart condition in June06. If Mary had purchased the insurance in October05 then the insurance should cover the condition because it wasn't present/known at the time the insurance was purchased. If Mary purchases the insurance in August06 then Bob's heart condition would not be covered, because it wasn't stable for the 90 days prior to the insurance purchase date.
Isn't that fun?