How much yarn you can spin onto a spindle depends on a variety of factors -- and yes, you can influence these factors.
For starters, do you have a "good weight" of spindle for the yarn you are spinning? If you're spinning fine, a heavy drop spindle will pull more than a light one (under 1 ounce), or than a supported spindle. If you're spinning thick, a light drop spindle or a support spindle will stop more quickly than a heavier drop spindle (over 1.5 ounces).
So, consider this ... as you spin, you add weight to the spindle. Sheila Bosworth of Bosworth Spindles said her goal with fine (laceweight or finer) spindling is to get 1/3 of the spindle's weight in fiber spun onto it. Why does she stop there? Consider that the heavier the spindle gets, the more "drop" you get in your drop spindle. The added weight may help with momentum on the drop or supported spindles, but with drop spindles it may also add to the droppiness ... reducing the enjoyment of the process for sure!
Now, you can limit the droppiness by using inchworm, or short-draw, spindling -- that way your spindle will not have the opportunity to pull fiber out of your hands, if the only pull is on yarn or less than a staple length (individual fiber length) of fiber. This also means things like silk are a bit easier -- the longer staple length lets you keep your hands a more "natural" distance apart. Short draw with silk is more like four-inch-worm than inchworm.
With thicker spindling, you are adding weight to the spindle fairly quickly. So, I'll choose to start with a spindle a little lighter than is comfortable (1.5 ounces, say, instead of the 1.8 ounces I find easier for spinning worsted weight) and know I'll be restarting it more often initially, until I have that first .3 ounces added onto it in fiber. After that, it keeps going well. I've found my thicker spinning will keep going and going until the spindle is approaching 3 ounces in weight -- which can help explain the 3 ounce "boat anchors" that were used in the past to teach new spindles. Yes, you can spin on a three ounce spindle. It just feels heavy! And, at 3 ounces, it really pulls, even against thick yarn -- so once again, you're using shortdraw and paying attention to staple length, to stop the spindle from dropping.
As for me, I usually plan to pack about 1 ounce of fiber onto a spindle. For me, that's "full enough". Here is my 0.9 ounce Turkish, with a 0.9 ounce singles yarn cake that just came off it -- the spindle was definitely approaching the "drop" zone, and inchworm was definitely in use as the spindle filled.
Then comes the plying -- for that, I take two spindles full and ply them together, so my plying spindles get 2 ounces packed onto them. Pretty darn full! I know you've seen this picture before, but it's really the best one -- here are the two skeins of sock yarn side by side. First sock-full, 1.8 ounces. Second sock-full, 2.4 ounces! It's a much fuller spindle, no?
Now, that's me, and my full spindle stories. I've seen some lovely full spindles on Criminy Jickets blog here and on Flickr here, here, here, here, and (my favorite)here. (And I'm sure there's more! See SpindleShots for a great collection of spindle photos!) I'd say that the fuller spindles use as much of the shaft as they can and still have room to flick the spindle, and have very lovely, even-looking cops. That evenness is important -- the main deterrant besides droppage is a bad case of the wobbles. The more wobbling you can tolerate, the longer you can keep piling on the yarn. However, wobbling eats up some of the twirl energy, so it's also less efficient. With practice and an eye for balance, wind your cop to keep it looking smooth and balanced. Have a shape in mind -- conical, football, beehive, and aim for that final shape of cop. Over time, you will find your cops being more balanced and your spindles holding more yarn.
For related topics, see:
How can I get more yarn on my spindle?
What is your favorite spindle weight?
When is the spindle full?
How much fiber do I spin to get 200 yards?
Collection of all of Ask The Bellwether's spindle posts.
Do you have a full spindle story? I'd love to hear it! Please post it in the comments on the blog or with a picture in the SpindleShots group on Flickr.