try also this expression:
first add an attribute to your object name it RPM,

if (frame == 1)
{ object.rotateY = 0 ;}
else
{ object.rotateY += (( object.RPM * frame * 360 )/(30*60))/frame;}

now you can key when your object will stop or go faster or slower base on the value that you enter in RPM attribute.

i got this stuff in highend3d.com