The temperature you cook at, and cook to, is what determines how long you'll cook it. You didn't specify what cut of beef you're using, but I'll assume one of the typical cuts, like top round or rump. These cuts don't have a lot of fat, but do have a large amount of connective tissue, so this makes them not only very tough, but also fairly easy to dry out. As a result, the braising technique is most often used.

To braise a pot roast:
1. Heat your oven to 250°F, or set your crockpot to low.
2. Season the meat on all sides with salt, fresh ground black pepper, cayenne, cumin, and minced garlic.
3. Melt butter or oil in a large, heavy bottomed pot, preferably one with a lid.
4. Saute yellow onions over medium heat until they're clear, then add the roast to the pan and turn up the heat.
5. Sear the roast on all sides, just until it's browned a little, then remove the pan from the heat.
6. Add beef or chicken broth until the meat is about half covered.
7. Add rough diced carrots and potatoes and celery, then cover and put in the oven.
8. Cook for about 2 hours per pound.

If you've got a probe thermometer, you can hold between 130-150°F (58°C), as measured by a thermometer inserted into the meat, for 5 hours. At this temperature, collagen breaks down, but the muscle proteins don't coagulate and shrink. This will give you a really moist and flavorful result, with little shrinkage. The interior of the roast will still be pink(because you haven't denatured the myoglobin), although the meat is fully cooked.

It depends on the type of roast, how "done" you want it, and if you are doing moist or dry roasting.

I wrote How to Cook Roast Beef which gives you information on how to dry roast it.
http://www.mahalo.com/How_to_Cook_Roast_Beef

I've also written how to cook pot roast, but it's not on Mahalo yet. However, there is no strict time limit for it. Cover and cook for six hours on high or eight to ten hours on low heat if using a crockpot. If doing it on the stove, cook for 3-4 hours

You can get more information on the rough draft of the page
http://greenhouse.mahalo.com/How_to_Cook_Pot_Roast

You've got this under "Pot Roast" so I'm going to assume that's what you mean.

There are all kinds of ways to cook pot roast. We happen to like a sort of Italian style at my house with balsamic vinegar, but the classic preparation is with onions, potatoes, and carrots. In some areas the classic includes turnips along with the potatoes, or in place of them.

The basic technique for the stove is simple (although I use a slow cooker): you brown the beef in a little oil. This improves the texture and flavor. Peel your veggies and cut them in chunks. Put them in the pot around the beef. Add a cup or so of beef broth; a tablespoon of worcestershire sauce; a couple of bay leaves, salt and pepper. Turn the heat to low and cook it for about four hours. In a crockpot, about 8 hours on low (more if it's unusually large).

The spices can vary: you can do all kinds of things, from ethnic things like Mexican or Italian, to interesting combinations. You can change the vegetables all around. One of our favorites has lots of garlic and cabbage. Another has loads of zucchini and peppers.

Just think of your beef roast as your own...paper doll. You can dress it up all kinds of ways.

Try this recipe:

http://www.ehow.com/how_4611950_tender-juicy-pot-roast.html