You will need one within a week of getting rid of them and it will cost you $60-$80 dollars. It doesn't matter if you've had them for a month or 10 years without needing one, as soon as you get rid of them, you'll need them.
Which is also where you should donate you're unneeded ones. that way, the day after donating them, you know where you can get cheap ones because you all of a sudden need some.
We should all get together and make a bank of these adapters, gather them all up in one place in every city so that when you have an electric razor with no power cord, you can just go down to the Adapter Bank and find something that fits