The farthest I usually go with device simulation is creating a reasonable waveform or data, or even just random data. A good simulation is helpful, but in the end it is worth nothing, so you have to judge how much time to put in it.
If you have created a project, you can go to project > properties, and create a new conditional disable symbol. You can make one called "PLC" and set the value to "False". Then, in your code create a conditional disable structure, and create a case for PLC being True or False. Do this every time you would talk to the PLC, and put your simulation code in the False case. Then when you get your device, put the actual code in the True case and go back to your properties and set the value to True, which will effect the entire project.
