How much yarn you get from a niddy noddy depends on the length of the niddy noddy -- so measuring it, if it didn't come with measurement information, is your best bet. A "typical" mass produced niddy noddy usually produces a 1 meter or a 4 foot skein (depending on which country produced it, I think).
But my favorite niddy size is one that produces a 1 yard skein, so I can just count loops and know how many yards I have.
You can determine the approximate skein size of your niddy by measuring the center bar -- if it is 1 foot in length, your skeins will be 4 feet around; if it is 9 inches in length, your skeins will be 1 yard around; and if it is 6 inches in length, your skeins will be 2 feet around (this is a great sampler size). The skein "size" is 4 times the length of the center bar, basically.
(posted by me this day on spinning_camp)