i'm not really understanding the question properly, but i interpret this as you are looking for the concentration of ammonia that would kill the frogs for what purpose? to hold off in changing the water until it reaches near critical levels? easier to just stick to a routine of changing water on a weekly or bi-weekly basis.

i have yet to run across any aquatic plant that releases toxins into the water column, so no need for worry there.

and a filter would not keep the water supply clean and perfect indefinitely. minerals are used up constantly from the water and need to be replenished. water changes are not just about keeping clean water.