Berghias live roughly 10-12 months. It takes them 3-4 weeks to reach shipping sizes which are 0.5- 0.75” usually on the larger size. 
We recommend a minimum of 3-5 to help them repopulate and get rid of your aiptasia faster.
These are bred locally here in Washington and are fed over 10 different species of aiptasia. 
Their diet is solely the pest anemone aiptasia, once aiptasia are gone these nudibranchs will slowly begin showing themselves during the day once the food disappears and they can be caught and rehomed.