DE Kabeer Gbaja-Biamila had arthroscopic knee surgery on Thursday. It's not surprising that a nine-year NFL vet is going to have some problems with his knees. He got hurt last season in the Thanksgiving day game at Detroit, but that was an ankle injury. The bad knee is something that just came up during the recent OTA practices.
Wikipedia says that it will take him a few weeks to recover which means he should 100% by the start of training camp on July 28th:
Most patients can return home and walk using crutches the same or the next day after the surgery. Recovery time depends on the reason that surgery was needed and the patient's physical condition. Usually a patient can fully load his leg already within a couple of days and after a few weeks the joint function can fully recover. It is not uncommon for athletes who have a beyond average physical condition to return to normal athletic activities within a few weeks.
Unfortunately the surgery is usually required when there is a ligament or meniscus tear. Obviously everyone's heard of the dreaded ACL or MCL tear, but the doctor might have just cut away the torn meniscus and his knee will feel fine with some missing. It shouldn't effect his NFL career, but could cause him problems later in life.