A “good” blade tip speed for GRASS cutting is about 200mph, or 17,500 feet per minute.
You get a 5’ bush hog, 5 x 3.14 (pi) gets you ~16 ft circumference or 16ft travelled by a blade tip in one revolution.
17,500 / 16 = ~1100 revolutions per minute for the tips to travel 17,500 feet in that...