My mother will have trabeculectomy in 2 weeks for glaucoma. what is success rate and how long would it likely keep the pressure down?

Generally good: The success rate is in the range of 85%. The definition of success is that the pressure is kept in a safe range after the procedure. It will keep the pressure down for years in most cases although there is a small regression in a few cases which might require a repeat procedure. It is still the standard of operative care for glaucoma.

Overall good: Ultimately recovery will depend largely on the post-operative course, and how much your mother's eyes heal. Trabeculectomy is still considered the standard and most efficacious glaucoma surgery to manage eye pressure for those with glaucoma recalcitrant to medical and laser treatment.