Todd,
Not that there's a minimum, but the longer the better. The more liquid that sits in the tubing below the level of liquid in the kettle, the more suction you get on your dip tube. [hehe, I said suction on your dip tube].
On a slightly more somber note, I found the limit of the hopstopper, DIY or otherwirse.
I had done three 5-gallon batches in my keggle using a mix of both pellet and leaf hops. I'd say the typical hop bill is something like 2oz pellets, 1oz leaf. I'd drain into my counterflow chiller and leave at most a cup of wort behind in the kettle.
On big brew day, I did my first 10gallon batch. The hop bill was 3oz pellet, 2oz leaf. The real difference is that I tried the immersion chiller this time so I added a ton of cold break to the filtering load of the hop filter. The runoff got stuck with about 6 gallons left to drain. UGHHHH. I wonder if using hop bags would have saved the day this time because maybe the filter can handle cold break but not cold break WITH 5 oz of hops.
Here's a pic of it in the boil with ~ 14 gals of oatmeal stout!! When you pull it out, it leaves a huge wad of hop crap in there. The mesh is very fine so it does take a while to drain completely.
