How much extra work to get a glider licence?
(I am assuming your question is asking about the FAA regulations.)

If you already have a PPL, then the requirements are modest. According to the Soaring Society of America:


  Additional Pilot Rating
  Holders of a valid FAA Power plane license with 40 hours as pilot-in-command need a minimum of 10 solo flights to qualify to take the glider flight test. No written exam is required to add a glider rating to a power license. In all cases, refer to the Federal Aviation Regulations for details on pilot licensing.