How much fiber you need for your project will depend much on how you spin. Experienced spinners tend to get a lot more yarn out of fiber, and newbies tend to spin denser, heavier yarn, requiring more fiber for the same yardage.
But, in general, here are my own guidelines:
3-4 ounces, spun sport or dk, will yeild a pair of medium ankle socks or mittens
4-6 ounces will give you longer or larger socks, or a long scarf
2 ounces spun into sport, 3 as dk or 3-4 worsted = a hat or cowl
1# spun up to light worsted = medium long-sleeved sweater or large shawl
1# spun sport to dk = you might squeeze a hood out of it as well
8 oz sport to dk = medium short-sleeved sweater or vest or medium shawl