I think the tube will hold 400-500 spent primers. However, I have a few LEE presses, and the spent primer tubes are not all the same length or diameter, so the quantity held will vary depending on these factors.
I eventually picked up a longer tube at Lowes or Home Depot and now run it from the decapping press into a 2 liter soda bottle. It is nearly full, and I'd guess there are over 15,000 primers in there of all sizes. Now I just have to find a scrap yard that will take them.