How common are invasions when playing as human?

It's likely that the PC version has the largest playerbase at this point: https://steamdb.info/app/211420/graphs/

For example, how many invasions could I generally expect during 10 hours of gameplay in human form (PC)?

There are multiple factors for this:

• Your network setup
• The level you're at relative to the area
• How long you hang around the area before killing the boss
• The area itself

Your network setup

Dark Souls is a peer-to-peer game. If you do not have the correct ports forwarded/opened on your network, you won't get invaded. It just simply won't work because invaders will be unable to connect to you (the host). When you are being invaded, the victim is the host.

The level you're at relative to the area

If you are super high level in a super low level area, or vice versa, you will not be invaded due to players generally not looking for victims within those level ranges.

How long you hang around the area before killing the boss

You can only get invaded if you have not killed the boss. If you know the game well, and rush the boss, then you won't get invaded nearly as much.

The area itself

People simply favor certain areas for invasion.