Unfortunately I've had all of the above. I've had clams die on me in the first 12 hours. However I answered with the most optimistic interpretation and that's looking at my oldest clams which are now somewhere in the 3 year mark. Actually my oldest clam, I've had for 3 years, but it was from someone else who tore their tank down, and I'm not sure how many years HE had it, but it was at least 1 year IIRC.
So, it's possible that some will hang on.
Unfortunately they're just incredible senstive things, so it's very easy to lose them.
But some do rather well.