I've heard that you should crank for as long as a minute though I've never done it that way.

I crack the outlet line at the main filter and crank until fuel seeps out (~ 5seconds). Then I crack all the injector lines and crank in 10 second bursts until fuel seeps out of each one, tightening those that show fuel. It hasn't taken more than 3 10 second cranks before there's fuel at every injector.

Sixto
91 300SE
87 300SDL
83 300SD