How much mass is needed to bend space-time continuum?

I picked up Astronomy and Physics as a hobby. I never studied these subjects in school and I'm not in college yet, but I'm trying to understand General Relativity. According to the theory, without matter, space-time is flat, but it curves when matter is present. I understand the idea but I can't find where it explains the WHY and HOW MUCH mass is needed to bend space time.