I've used as low as 2 tsp/half gallon--but I don't know for sure that there were ich spores on the plant. I would leave it in for about 30 second if it's a broad leaf plant, around 10 for more delicate plants. Check in the Planted forum--there's a thread on how to use bleach to sterilize. Honestly, though, if I suspected a plant was in with fish that had ich, I would just put it in a tub of water with no fish for 3 weeks--I wouldn't heat the tank to 86, but 3 weeks is long enough that the parasite would have died for lack of a host.