You usually figure out how much you need for anything by yardage, not weight. And the yardage is totally dependant on the grist of the yarn (How much fiber is packed into each little length).
Just a rough rule of thumb, though. (and it's very rough)-- 1/2 a pound will give you enough yarn for a hat and gloves or mittens, a pound will give you enough for a BIG scarf and maybe a hat to match, 3 pounds will make enough yarn for a pretty big sweater
These are just sort of rough estimates you might use to decide how much of a particular fiber to buy. If you're spinning laceweight, you could possibly get a 60" x 60" shawl out of 2 or 3 oz of yarn.
Best thing is to find a pattern you like, figure how much yardage it requires and then spin a similar yarn to the number of yards you need. It really isn't very hard to do. You'll find that your handspun is real knittable.