How long would I have to treat them. The bottle says 3 days, but I
thought
maybe a week. I am also mixing their food with tetracycline because their
tails are shredded.

This stuff sure acted fast. The rot and the ich, I mean.

Like almost everything opinions seem to vary. Most sites agree that
treatment must continue past the 'visible' stage. I have seen often quoted
the time period of 3 days past. If your malachite green also has formalin,
formalin is toxic. I have treated for 10 days alternating 1 day treatment
and next day 50% water change and it has not returned. It is possible that
the suggestions of treating for only 2-3 days are actually effective because
the medication remains in the water. Additional suggestions are to keep
your tank dark because light deactivates malachite green. A temperature of
78-80 will also accelerate the life cycle, increasing the probability of
killing them during the 'free swimming' stage. You also should remove any
charcoal as this removes the 'medicine'.
In any case remember that the cysts falling from the fish is a part of the
natural life cycle of ich and is not indicative of a cure. HTH
Bill