F = Failsafe (safety integration related tasks) run periodically.

To capture the runtime of an F block you would need to disable interrupts, start a runtime timer, execute the F-block tasks, capture current runtime timer data (to accumulate cycle time), increment a counter (to enable averaging F-block cycle time), and then enable interrupts.

To predict the F-block runtime offline may require more calculations and perhaps even some details that Siemens may not be willing to share.

I recommend submitting your needs to Siemens Support to see what they can offer to help you solve your specific problem(s).  Use the "Support Request" link in the gray bar above these online forums.