It depends on the PLC. On SIMATIC PLCs the bus cycle is not equal to the PLC cycle time. The bus will normally run as fast as possible, and the PLC too . If you have a short PLC program and a lot of masters and slaves, the bus is slower than the PLC, if you have only a few slaves and a big PLC programm, the bus is faster than the PLC. You can't "guess" the PLC cycle time from the bus cycle time. All above for normal DP data exchange mode.