The majority will die of starvation within a year, but often after lingering many months (I think 6-9 months is considered the standard period for starvation). They require a specific particle size so that even if food is added, or attempts are made to feed it, they may not be getting specifically what they need. They will move and should be left where they wish to go, which may mean behind your rocks and out of site. So there needs to be actually a fair effort involved in keeping them. I would not say they are easy or hardy.
If you google "rob toonen flame scallop" you should come up with an excellent article that I have quoted on this site a few times. Also I guess you could search the site for "flame scallop" if you haven't already.