If you're starting with a known velocity and cutting THAT barrel, 20-30 fps for each inch you cut. If you're working with 2 different barrels there is no way to know until you shoot them. The rifle with the 22" barrel could be 50 fps faster than the one with the 24" barrel. Or the 24" barrel could be 150 fps faster than the 22" barrel. Or they could be the same.
My 20" Tikka CTR is the fastest 308 I own, shooting loads 25-30 fps faster than the two with 22" barrels. It isn't unusual to see differences of over 100 fps between barrels the same length. My 22" Winchester 30-06 is always 60-90 fps faster than my 22" Remington 30-06. It varies depending on the exact load.
