PC to router - 1ms or less. Generally, unless you're talking a very long line and/or a lot of hops within the network, they should be very low.
Ping times rise with latency (line length, quality, hops, and number of people using the line). If you have four PCs all using the line from a router to the Internet all at the same time, latency will rise, but that's more an external thing as generally the internal network is far superior to the broadband speeds available. Most internal networks are good for at least 10 Meg and probably between 50 and 100 Meg (bits) depending on the ethernet node quality, cable quality etc. Gigabit kit is good up to 1000 Meg in theory.
One other thing. Some software programs or processes can cause latency on a network, but I would expect the software to be knowingly in use, such as downloading or gaming. Best to not run anything when testing latency.
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