you can definately do it. a lot of people keep clone mothers alive for five years or more. whether or not it's really worth it is another question. thats a long time to wait for a harvest. why not just grow a few plants at a time and put them outside for harvest each year. also, a sativa can get way too tall for most people to fit inside if you veg it a long time. if you have high cielings, which it sounds like you do, it might not be an issue. but once you get it outside and it shoots up at the beginning of flowering, its a lot harder to keep a 20 foot plant from being seen outdoors than a five or six footer. it would definately be a lot of fun to grow a monster like that though.