Roan Art said:
Further to what Boesemani Fan is saying: whether or not it'll be lethal will depend on the fish. Some are more "hardy" than others. The best approach is to assume anything over 0 is possibly lethal and deal with it from there.
Roan
Ditto- in addition to that is what kind of fish it is, as some fish will and can utilize air from the surface of the aquarium. Additionaly, it can depend on your pH, as it is mainly ammonium at pH under 7.0, and that is far less (some argue completely) harmless to fish. In addtion to that, fish that live on the bottom will experience more ammonia than those at the top, in most aquariums.
Considering what the norms are for most fishkeepers, that information is not all that helpful, as most of us are over 7.0 and don't have tanks full of Labyrinth fish... but you should be hypersensitive to Ammonia during cycling if you have bottom fish in there.
Perhaps a better question is not what level is lethal (as that is different for a lot of fish), but what level should my ammonia be kept under for safety during cycling. Keeping it low can be hard during cycling, but for the most part, that is one of the only times you will have to worry much about Ammonia, unless a huge Apple Snail dies hidden somewhere in your tank.
I would keep it under .25ppm if possible, but I would not be changing more than about 25% of the water daily to maintain that- unless you continue to creep up toward or above .50ppm. If your levels are higher than that with that schedule, you are doing something else wrong, IMO- overfeeding, have too many or too large of fish (for the cycling purpose/process), or fish which produce too much waste, etc...