If you give a follower an enchanted weapon, does it lose charge as they use it? Or does it never decrease?

Yes, its charge reduces like normal. I also know when Mjoll's staff is depleted when she switches to an alternate weapon. I then have to take it from her, recharge it, and give it back to her.

i gave aela legendary elven bow with fire enchant she was rocking!!!!

Some do and some don't. Example, I gave an enchanted soul capture bow to Lydia along with some Grand soul gems. The bow never depletes, but she fills the soul gems with each kill. Probably a bug, not sure. She also carries an Ebony vampire sword which needs re-charging constantly. Hoping patch 1.3 will fix some of the enchantment issues. :confused: