If the actuator arm is six to ten inches long, then Owain's answer can be converted to a force in the range 1,000 to 3,000 lbf for that particular aircraft. This is the capability, ie the force available to meet all eventualities. The actual force required under normal operating conditions may be much smaller.
Every design will differ and I suspect assessment of the required force is very much a wind tunnel issue.
The use of slats and their effect in extending the Cd curves are well described here:
Anatomy of a STOL Aircraft