@Hauke Has a very good, in depth explanation, however it can be simplified a bit. Surface speed boils down to Circumference x RPM. So going at our max speed of 2700deg/min, we divide by 360 to convert to RPM, since 1 rotation = 360 degrees.
deg/min ÷ 360 = RPM
2700 ÷ 360 = 7.5 RPM.
To find your maximum surface speed for your object, take the diameter you measure with calipers and multiply buy pi, or 3.14 for simplification to get the circumference . For say a 100mm object (mug maybe?)
d x π = Circumference
100 x 3.14 = 314.
Then multiply together get mm/min surface speed.
RPM x Circumference = mm/min
7.5 x 314 = 2355
Which matches what @Hauke got (minus the rounding error using truncated 3.14). This will be the max speed you can engrave across the surface. 
TL;DR Version;
d x 3.14 x 7.5 = Max Speed in mm/min.
Measure the diameter of your object and swap for d