I think I have the facts right.
2 liters of water every other day.
2 plants in 1 - 5 gallon buckets.
No nutes used yet.
Plants around 6'' tall.
No LST or topping during grow.
Knat infestation.

To answer your question your soil is probably saturated currently. Lift the bucket and note the weight. When the bucket weighs 50% less it is time to water. This will be close to 7 days judging by your plant size.

The slow growth is due to low light, you have solved that with 6 cfls

Your plants are stressed so hold off on nutes till you get them growing and healthy again.

When you get them growing again consider performing LST. read up on it in the FAQ section you have a little time before starting it.