No shit, there I was, leading a team of 15 teenagers on a backpacking trip in the Mokelumne Wilderness with two other counselors, and "Ziggy" (one of the staff, NOT a teenager) had kitchen duty. That night's menu: spaghetti-- an easy backpacking food, and Ziggy was a confirmed bachelor with years of both outdoor education and bachelorhood under his belt. I'm setting up camp with the kids and stop by to check on dinner, and my jaw drops open.

Ziggy has two pounds of pasta in a pot, with about a quart of water. The water is no longer boiling.

    "Ziggy, are we low on water?"
    "No, why?"
I point to the pot. "That's not enough to cook the spaghetti."
    "Sure it is. That's the way I do it at home."
    "Yep. All the time."
We all sat down to perfectly cooked bachelor spaghetti, an hour and ten minutes later.

Granted, we were at 6000 ft (1800 m) altitude, which will make cooking time longer, but six times longer than the package recommended!!?! No. The reason it took so long was that there was not enough water to cook the spaghetti efficiently.

Bachelors: Use the amount of water recommended on the package.

I know you're broke. Water isn't expensive enough to skimp on here. I know you're a competent adult who doesn't need to read directions. Hey, mister hunter-gatherer-- Use the water. Yes, smaller amounts will come to a boil in a shorter time, but if you want "perfect" spaghetti, you need to immerse the dried noodles in a lot of water. Why?

a generous pot of rapidly boiling water is helpful for several reasons: it comes back to a boil faster when you add the pasta; it makes it easier to submerge long, rigid pastas like spaghetti; and it helps to reduce sticking slightly by quickly washing away the exuding starch from the pasta surface.
-Fine Cooking Magazine
On the off chance you don't have the package with you, here's some recommended amounts:
    1 liter per 100 grams of pasta, or
    24 oz. package - 7 quarts
    16 oz. package - 5 quarts
    half a 16 oz. package - 3 quarts
    one handful - 2 quarts

belgand adds:
My personal rule of thumb on this is to fill the largest pot in the house such that with the pasta added there will be about 2 or 3 inches of space at the top. Of course this also means knocking a minute or two off the cooking time to prevent it from overcooking, but that's almost another topic.

sneff adds:
I have to agree with belgand here. I always use the largest pot in the house, which in my casa is 20 litres (5 gallons) - even if I am cooking pasta for one. Or to put it another way, there can never be too much boiling water -- however too little is unwaveringly a disaster.

A few tips you may want to share for those scared off by the amount of time it takes to boil this much aqua: Always fill the pot from a hot tap. Find the hottest burner or hotplate on your stove (there always is one) and use that for boiling. Cover the pot with a tight-fitting lid while waiting for the water to boil - but never cook the pasta with the lid on. And lastly, add the salt (you added salt to the water right?) after the water has come to the boil. Higher salinity leads to longer boiling times. Following these rules, I can get 18 litres of water from tap to boil in 6-7 minutes.

I also tend to turn the heat down a small amount once the water returns to the boil after the pasta is added. Sometimes, a violently rapid boil can cause pasta to stick to the base of the pot, and also cause tubular pasta to crack. I find a nice, polite rolling boil best.

Razhumikin and begland reply: Never drink or cook with hot tap water.

Don't use a mere one quart of water if you're cooking spaghetti, unless, of course, your idea of perfect bachelor spaghetti is a gooey sticky and partially firm, partially soft clump of noodles (because you didn't use the volume of water required to wash away the dissolved starch from the cooking pasta)-- and if it is, well, offer me a beverage and I'll be on my way.