Actually the regulations state that any add-on category rating only requires 15 hours in category.

When I was a DPE I tested a few SEL to Glider CFI candidates, all of them wanted to become more proficient gliders pilots. Many of them used the add-on process as a renewal for their CFI certificate and never intended to teach in gliders. It's likely they have been teaching for at least two years before adding the glider rating.

They must also have received instruction from a glider certificated instructor before taking the test (though no endorsement is needed for the first attempt). They must also pass a knowledge test (short version, 25 questions) on soaring principles and topics.

The regulation also states that they must pass a Commercial Glider test first, before applying for the CFI-G.

I recite all of this to make the point that this is not a casual, single weekend process, and I think very few would actually apply for the test with only 15 hours.

Mike