How long does it take for a crocea clam to attach.

[Tasher80]

I got a crocea clam two weeks ago and had it on the sand bed then read they like to be on the rocks higher up. So I put a small flat piece of LR under it so it will attach to that and not my larger pieces. It fell off today so I put it back on but in a more secure spot. How long should I expect it to be before it attaches??