Well, it does matter, as some products may have the function built in. But as suferb noted, if you assume that's not the case and you want a 'generic' approach, then I would do as suferb suggests:
For the PLC, use the inital Scan status (whether an internal bit in the case of AB, or something you make-up) to clear a register. Then update this register with a pulsing timer at the resolution you need. Don't go too low or you may outrun the size of your register. Use a floating point to avoid this if possible.
For the SCADA, you need to define whether you want the SCADA only or the combo of the SCADA talking to the PLC. In some applications you may want only the fact that the SCADA is up and talking to the PLC, not the fact that the SCADA application is running. If you want that the PLC/SCADA combo is up and communicating, then use a startup script in your SCADA to initialize a PLC register as above and use the same counting routine. If you only care that the SCADA app is running, do the same but with an internal register to the SCADA package and an always running script that periodically increments the register.
