Sdcard stated bandwidth at max working hours
If you want to be 100% sure, you should either ask the manufacturer, or ask for a /find a datasheet that gives you some nice graphs for comparisons.
If they are honest (which I doubt), since they give you just the hours, it should not matter how fast you write/read from the card, but just how much time you use it.
I would reccomend you measure it by taking the worst case (Lower read/write speed multiplied by the hours), if you want make a professional estimation.
You can see in SanDisk's datasheet that there is a register that tell you how much life is still in the card. It also tell you that you can still keep using it, but it is not guaranteed it will work just as good.

I am also pretty sure they (Sandisk engineers) do not know the exact answer to your question either.