Going by the C.A.T.S data for Pro/Elite bowlers,They are averaging 19+ mph at release.
On average a ball will lose about 3 mph from release to the pins ( depending on conditions and coverstock, also the ball path straight vs curved). So at 45' you should at about 17 mph.
According to the Usbc ball motion study, 17 mph at the pins is the ideal speed and 21 mph when released.
The thing to watch though is speed variance, The Elite/Pros only vary by .38 mph over ten shots.