There is no typical initial velocity.
It really depends on the site conditions and what is instigating the rockfall. For example, if it's from weathering and blocks detaching from a slope surface, then a small initial velocity is warranted (a small velocity that gets the rock moving). Some people simply drop the rock from a small height with no initial velocity. It is up to the user.
If the rockfall is seismically induced, then values on the order of the peak particle velocity of the slope may be warranted.
One thing to do, though, is to make sure you look at varying the initial condition (velocity, drop height) either statistically or by simply varying the initial conditions to make sure you have some idea as to how it affects the results (run out length, barrier design, bounce height, etc.)