The spindle is full when it gets hard to spin on -- the yarn slips, or there's nowhere to twist it. Or, it's full when you run out of roving/fiber. This is true for any type of spindle -- top whorl, bottom whorl, turkish, support.
It's okay to cover the bottom of a turkish spindle; I lose the "point" at the bottom of mine all the time from winding on beyond that distance.
(posted by me on spindlers on 11/12/2001)
And I've learned a few things since then ...
Consider ... you start with a 15 gram spindle to spin singles for a sock-weight 2-ply. How much fiber do you expect to get on it before it becomes difficult to spin, due to the added weight on the spindle super-drafting your fibers for you ... said another way, how heavy of a spindle do you want to spin sock-weight yarn on? Something to consider.
A great "rule of thumb" I learned on spindlers from Sheila Bosworth of Journey Wheel (and Bosworth spindles!) fame is to aim to add 1/3 the spindle's weight to it in fiber. So on that 15 gram spindle -- that would be just 5 grams of fiber! A mere handful.
On the other end of the spectrum ... both a local friend of mine (Debbie) and an internet friend (Abby) stuff as much as they can on a spindle. Debbie puts up to 2 ounces, on her favorite 1-1.5 ounce top-whirl spindles. And I believe Abby once posted she had 4 or 5 ounces on her favorite 1-2 ounce bottom whorl spindles. Now that's full! Granted, neither was spinning sock-weight singles at the time.
So, something between the advice of 1/3 the spindle's weight and triple the spindle's weight seems to be the "norm". I guess the spindle police won't be handing out tickets anytime soon for over-filling a spindle!
And me personally? I think I use something like Sheila's guideline generally for my finer spinning, and put about an ounce on a spindle when I'm spinning singles to ply into worsted yarns.