The regeneration can start right away, and you will see a little bud where the old arm was. But it can take several months to a year for the arm to be normal-sized. Limbs regenerate faster when the starfish is healthy and finding enough good food. If conditions are bad, the regeneration will take longer.

There is no reason they could not regenerate limbs in a lab, but it takes some effort to keep a good marine aquarium.