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I'm unsure you can accurately calculate throughput for a "typical" user LAN vs. a real user LAN as they often differ so much. You need to account for the role of each person on the network, i.e. what's their expected data usage. Additionally, you also often need to account for how hosts are configured to utilize the network.
For example, how much data a user will push or pull generally differs much by job role. A manager, programmer, admin, data entry, CADD job role all manipulate different "volumes" of data during the same timeframe. How much of this data volume crosses the network depends on whether, on one extreme, most of the data resides on the local host or, on the other extreme, whether most data resides on a different host.
Although you've asked about throughput, modern LANs often vary in their needs regarding latency. For example, the network requirements to support host to host bulk data transfers vs. host to host real time data transfers impact "throughput" requirements.
Regarding LAN vs. VPN, the latter tends to add overhead which reduces available throughput. Often, but not always, the loss throughput capacity, for overhead, isn't an issue on a LAN. However, VPNs also often introduce additional processing requirements, and throughput might be significantly reduced because of this. VPN throughput reductions very much depend on the kind of VPN and the supporting equipment.
