How many devices should be connected to a network?

A long-term "guest" keeps her laptop, iPhone and tablet all tuned on and connected to my home network Wi-Fi all day and night, not doing anything meaningful, just surfing the net, tweeting and looking at videos. My significant other and I use just one device at a time. Is our guest's "over-usage" slowing down our network, not to mention wasting electricity? Should we find a polite way to address this, or should we just let it go?

If the guest has all of their devices connected to your network at one time, then it should not waste any electricity. The only electricity being consumed would be to charge their laptop/iphone and tablet, so you may want to regulate that. The only time your network would slow down would be if this "guest" streams movies or decides to download any content. Just looking at webpages or going on facebook shouldn't affect speed. If they are staying there for a long time as you mentioned, then I would recommend having some sort of "fee" for using your connection. Hope this helps.

While these devices may be using up some of the bandwidth on your network, I wouldn't worry too much about a reduction in network or Internet speed.

Most networks and hardware today can handle quite a few devices connected to them, such as is the case with coffee shops, schools, and even people who simply have many Wi-Fi enabled devices, provided nobody is doing anything particularly data-intensive on the network.

So, to answer your question, I think you would be alright just leaving things the way they are, unless you notice a significant difference in your network or Internet speed. But, having a few devices, like a computer, iPhone, and tablet connected to a network shouldn't cause any significant problems, especially if they are only browsing the Internet, watching YouTube videos, etc.

Best of luck and I hope I helped you!

Not more than about 40 per access point, otherwise you start to get congestion.
If your uplink is much slower than the LAN, then of course you have a bottleneck, and some devices "just turned on" can still be on the internet doing updates or streaming video that no-one's watching or reloading adverts on unattended web pages.

With a wireless N network, your router should be able to handle close to 250 devices. Your slowdown will be your ISP connection, not on your network.

Unless your guest is simultaneously watching videos on their iPad, iPhone and laptop, you won't see much in the way of problems.

Devices such as phones, tablets and laptops are incredibly energy efficient, they are designed that way to use as little battery power as possible.

Sounds like you have an unwelcomed guest, while there might not be an issue network wise here, you may want to have a discussion with your guest on what is truly bothering you.