How long do you buy domains?
It depends on a few things really.
If it's a new idea I have, I'll register them for 1 year, if the project is still going strong after that year I'll do 3 or 5 years at a time.
I wasted so much money on registering domains for several years and them not being used, so now it's just discapline not to do that.
There's a few domains I'll keep on my books like preppers.org.uk just because I like it, short, snappy, popular niche. So I register these yearly but any projects that I've had for more than a year that are doing well and I still have a passion for I'll register for as much as my budget will allow.
I think my personal business domain name was registered for 10 years and I'll add to it as and when I can.