It depends on if your fiber prep is worsted or woolen and if you are spinning worsted or woolen- worsted spinning and prep (such as combs and tops) is much denser and how many people learn to spin. The yardage will be less than many commercial yarns, which often have elements of woolen prep or spinning in them. (exception being some really sturdy sock yarns).
I usually spin woolen prep and woolen spinning with a long draw for dk and worsted weight yarns, and woolen prep and worsted spinning for sock yarn (or versus, but I prefer to card on my drum carder).
Most people will say start with 4 ounces of a fiber to begin with, which for wools in roving form is usually about right- it will give you a pair of socks, some wrist warmers, or part/all of a hat (depending on if its single or plied and how tightly).
For more exotic or $$ fibers though, I say just go with 2 ounces, unless its something like angora or angelina, which you usually blend anyways with a couple ounces of another fiber. Two ounces is enough to play with and still get some yardage out of. Combine 2 or 3 samplings and you easily have a striped hat, scarf knit the long ways, etc.