How long should membrane remain with bone graft?

Depends on membrane : Collagen membranes are absorbed and become part of the graft. Gortex or ptfe membranes stay in anywhere from one to 6 months usually.

Bone graft membranes: Bone graft membranes should remain undisturbed after grafting to protect the underlying graft and keep out bacteria and saliva. The exact length of time will vary depending on the type of graft and location in the mouth. Over extraction sockets, they should remain at least four to eight weeks while the gum tissue heals over the site. Most collagen membranes will take 3-4 months to resorb.

Depends on graft: Membranes are made of different materials and may be resorbable or non-resorbable. Depending on the extent of the bone graft needed, they can dissolve in a couple months, or until the doctor removes it after the bone graft has healed. This could be 6 months or more. Keep smiling !