In answer to your question, if thats the type of anemone that those clowns host to, then the time they've spent without the anemones should not matter. Read the stickie in this forum about what clowns host to what anemones.
I recently bought some captive bred juvenille Maroon Clowns (so they had never seen an anemone that I know of). First thing they did was host to my power head (its a nice powerhead? ha), then when I moved that they immediately dived into my BTA and haven't left (I'm hoping they'll venture out one day).
I think its much more instict than anything else, in other words, time away from anemones is not a factor.
Can you post your tank specs and water conditions before you buy a carpet anemone? They are some of the more difficult anemones to keep and need very strong lighting. They can also grow up to 20 inches. Good luck!