How long is a typical CouchSurfing stay?

Some interesting stats were done in 2008 which found that the average length of stay is 2-3 days. Indeed I've heard of many being turned down for one-night stays - hosts often host because they want to get to know people, not just be a couch for the night.

As for longer stays, often you'll find it's cheaper and more sensible to find someone with a spare room which you can rent - even at so-called 'mate's rates' - ask your short-term hosts initially if they know of anyone with a spare room or similar. A hostel is fine, and I've done several weeks in a single hostel before, but if you're working I assure you it can get old fast when the rest of the dorm comes back from partying at 3am and you've got work at 7 ;)

Is CS intended for short (1- or 2-night) trips, or is it not unusual to stay for a whole week or longer?

CS is "intended" for both short and long stays (the host can freely decide), and very long stays are not unheard of, but I agree with the other answers that 2 to 3 nights is the most typical.

Which kinda makes sense if you think about it. How you'd react when someone you don't know at all asks if they can stay at your home for, say, the whole week? That's kind of a big promise to make, when you don't know how you'd get along. (Yes, even if you've read many positive reviews about that person.)

I was asked once if I could host for 5 days or so, and I remember I felt kind of reluctant to make such promise up front, even though the person seemed like a perfectly good guest.

So, indeed, a couple of nights is way more typical, as a starting point at least. The host can always extend the offer if they hit it off well, and in fact I think that's quite typical too!

I've hosted more than 50 people already living in different cities and I can tell you, depends on! If you really think that you have something to offer and share between both I think you can ask from 3 to 5 days. Have in mind that you read the profile of your host and you think you gonna share something, like teach him german, or how to cook. And he has in his profile something like, "I love cooking".

The experience itself is more important than everything. I recommend you to always ask for 2 or 3 days. If you get used to each other and you have good connection, ask for more. They will understand.

A last advice, try to be clear when you ask, for example, if you really need 5 days at someone place, tell the reason. For example, you want to visit something that open just weekends. Also be clear saying that you already booked your bus or flight to the next city! Then the host will not see problems hosting you.

In the last case, ask for 3 days and stay the other days at hostels or ask for someone else.