I keep my tree up 'way past Orthodox Christmas - we've had Valentines Day trees and Easter trees: I flavor the water with MiracleGrow (about a quarter to a third of what they recommend for indoor plants) and the damned things just keep going on happily! Still have a nice scent, and I've gotten better than 2" of new growth on 'em some years before we finally get tired enough of 'em to toss 'em outside.
As long as they keep their needles well (Fraser firs and Balsams are preferred) I've no problems letting them grow.
Good idea about keep the trees green and growing. I never thought about putting Miracle Grow in the water, but it makes sense. Thanks for the hint.