How many days did Paul stay with Cephas on his first visit to Jerusalem, according to Galatians?

Answer: Fifteen

Only after three years did Paul go up to Jerusalem to confer with Cephas (Peter), and he stayed with him fifteen days.

“Only after three years did I go up to Jerusalem to confer with Cephas, and I stayed with him fifteen days.”— Galatians 1:18 (BSB)