Good point, since piash didn't mention what CPU he is using. The higher-end Beckhoff CPU's like CX1020, CX2020 can run 0.05 ms cycle time (w/ reasonable amount of PLC code and I/O). I usually run them at 0.1 ms cycle since fast enough for the A/D rate of most analog modules (10 kSps). I ran timing tests on a CX5010 (Atom CPU) and found it exact at 1 ms cycle, but lagged proportionately at faster cycle times, so don't exceed 1 ms for critical timing tasks. For reference, I recall the lags were ~0.05% at 0.1 ms cycle. I ran these tests against a high-speed data recording system, with the PLC toggling a digital output each cycle. I recall the BX series processors (ASIC, non-Windows) can't run faster than 10 ms, which is typical of common A-B and Siemens PLC's. Years ago, I tested a CX1000 (now obsolete) at 1 ms cycle and found it lagged 30 ppm, which is near PC clock spec's, but still unexpected. At 10 ms cycle, it was exact.
Re the fastest PLC's, I looked at Beckhoff's EtherCAT and B&R Automation's Ethernet Powerlink in 2005 because they were the only PLC's which could respond to an analog input with a digital output in <2 ms. I had a strong internal push to use A-B if at all possible, since my boss claimed "industry standard". We tested a Control-logix w/ fast A/D module. I recall we could get it down to ~4 ms, but only with a few analog channels. I also noticed National Instrument's ads for their "fast" CompactRIO, but they gave no specs. I am glad I chose EtherCAT since it became widespread. Years later, NI gave specs at a seminar saying CompactRIO could read analog at 1000 Sps for 4 channels, or 100 Sps for all channels (10 ms updates), using their built-in functions. I commented, "that isn't what we call 'fast'". They said one might do better by writing custom FPGA code, but that seemed a daunting task. NI adopted EtherCAT for their Compact module expansion chassis. I think B&R is still the only competitor to Beckhoff for high-speed PLC's. BTW, A-B salesmen have made the most absurd claims to me about Beckhoff, saying it can't be real-time since "it runs in Windows" (not true), so process such comments appropriately.