Q.How Long Should You Keep a Christmas Tree Indoors?
I have had my tree since late December. The needles didn’t drop off like most trees after about 2-2 1/2 weeks. This has been the most unusual experience that has turned into a project! We keep it watered, away from heat sources, good indirect light and ventilation. I see clear sap in a few places on the trees and on the buds growing at the ends of some of the branches. The branches are stiff but it still
seems to be living. Should we be misting the tree or thinking about a sad farewell? Oh yes, we took off the decorations off long ago, I never use lights on my trees, and it is still in the tree holder.

Certified GKH Gardening Expert
If it is a cut tree without a rootball, then it is essentially a giant cut flower. It is impressive that you have kept it alive this long but as it has no root system,it will eventually die. But there is no reason you can't keep it for fun until it does die.
If it has a root system, then now would be the time to think about acclimating it to the outdoors and finding a place to plant it outside. Keeping them indoors only gets them through a short period and they will not live long term indoors. These articles will help you with acclimation and transplanting:
https://www.gardeningknowhow.com/houseplants/hpgen/move-houseplant-outside.htm
https://www.gardeningknowhow.com/plant-problems/environmental/learn-how-to-avoid-and-repair-transplant-shock-in-plants.htm