How long will ground beef stay good after being frozen?

From Freezing and Food Safety published by the United States Department of Agriculture:

Is Frozen Food Safe? Food stored constantly at 0 °F (-18 °C) will always be safe. Only the quality suffers with lengthy freezer storage. Freezing keeps food safe by slowing the movement of molecules, causing microbes to enter a dormant stage. Freezing preserves food for extended periods because it prevents the growth of microorganisms that cause both food spoilage and foodborne illness.

Does Freezing Destroy Bacteria & Parasites? Freezing to 0 °F (-18 °C) inactivates any microbes — bacteria, yeasts and molds — present in food. Once thawed, however, these microbes can again become active, multiplying under the right conditions to levels that can lead to foodborne illness. Since they will then grow at about the same rate as microorganisms on fresh food, you must handle thawed items as you would any perishable food.

Trichina and other parasites can be destroyed by sub-zero freezing temperatures. However, very strict government-supervised conditions must be met. Home freezing cannot be relied upon to destroy trichina. Thorough cooking, however, will destroy all parasites.

Freshness & Quality Freshness and quality at the time of freezing affect the condition of frozen foods. If frozen at peak quality, thawed foods emerge tasting better than foods frozen near the end of their useful life. So freeze items you won't use quickly sooner rather than later. Store all foods at 0° F (-18 °C) or lower to retain vitamin content, color,

It depends on two factors:

1. How it was packaged prior to freezing. I have gotten 1 pound blocks of ground beef from my butcher, which are vacuum sealed before flash-freezing in shrink-wrap. I have pulled these from the freezer over a year later and found them to be perfectly fine. On the other hand, if it is typical grocery store Styrofoam tray wrapped in cellophane then 4 months is about the outside limit before freezer burnt beyond usefulness.

2. Fat Content, those packages I got from the butcher where better than 90/10 (more like 93/7). Fat can go rancid, even while frozen (though it does take much longer, I suspect this is part of what @ElendilTheTall was referring to as "properly" bad.) The higher the fat content the faster it can go bad.

If after thawing you are not confident of its safety then yes, throw it out. A fresh pound of ground beef is much less expensive than a trip to the ER for food poisoning.

The USDA recommends four months. It won't be unsafe after that time per se, but it will very likely be freezer burned, giving it a weird and unpleasant texture.

Eventually it will go 'properly' bad - I don't think I'd risk food poisoning over a few bucks worth of ground beef. Throw it out and buy fresh.

I had ground beef in the freezer for one year in a Zip Lock bag and it is in perfect condition. Don't through out food unnecessarily if it has been kept frozen.