Geometry and Trigonometry is about 15% of the Math section, which works out to roughly 6 to 8 of the 44 questions. It is the smallest of the four Math domains, and also the most closed-ended, which makes it the fastest domain to prepare completely.
Being the smallest domain is what makes it the most efficient one to finish. The content is closed-ended, the reference sheet covers most of the formulas, and unlike Advanced Math there is no long tail of unusual question shapes. Complete it early and stop revisiting it.
The full picture, with the surrounding detail, is in SAT Geometry & Trigonometry: The 7 Questions Nobody Drills.