Siemens SIMATIC S7-400H Manual page 69

Calculation of the max. scan-cycle time extension
We recommend the following formula:
Maximum cycle time prolongation = 10
classes > 15)
The final time is determined by the process state and the communication load on
your system. This has to be taken to mean both the absolute load and also the
load in relation to the size of your user program. You may have to correct the time
if necessary.
S7-400H Programmable Controller Fault-Tolerant Systems
A5E00068197-04
Link-up and Update
(maximum blocking time for priority
5-25