More time is not better. Percussion therapy follows a curve where the benefit flattens out quickly and discomfort starts to build.
One to two minutes per group
That is the working figure for most muscle groups. Keep the gun moving slowly across the muscle rather than parked in one place.
Fifteen minutes total
A full session across several muscle groups should land somewhere around ten to fifteen minutes. Beyond that you are usually just irritating tissue.
Areas to avoid
Keep off bone, joints, the neck front and sides, the spine itself, and anywhere bruised or injured. If something hurts sharply, stop.