I found a few more sites with dosages-
This one is from the Belle Island Aquarium-
Levamisole (13.65% injectable) - 2 ppm (1.1 ml per 20 gallons)
This medication chart suggests-
2 mg/L for 24 hours ONCE. Most effective against larval nematodes, swimbladder worms. no increase in effectiveness with higher dose.
In the book, Fish Disease: Diagnosis and Treatment by Edward Noga pg 288 (you can login to amazon.com and search inside this book, search "levamisole" or "288")-
The dosage they give here is 10mg/liter or 38mg/gallon of water for Anguillicola (nematode that lives in the swimbladder of eels) and other suspectible nonencyted nematodes. No mention of treatment time.
So far from what I've collected-
1mg/l (38mg/10g) 1 to 2 days for skin and gill flukes
2mg/l (76mg/10g) once per week for 3 weeks for Camallanus Nematodes
10mg/l (380mg/10g) for Anguillicola.