The maximum amount of nitrates that you should have in a tank really depends on the type of livestock that you have in the tank. Fish can handle higher levels then corals and inverts. Most mobile inverts can handle higher nitrates then corals.
There really isn't any exact data on the max allowable nitrates as each animal is different here are the general guildlines I've come up with based on my experiences over the past 6 years keeping various animals
For a fish only system without any inverts (including snails, crabs etc.) can handle around 30-40ppm before the fish start to stress out easily and lose some colouration.
Systems with mobile inverts and the most hardy of corals can around 20-25ppm. If the levels are above that for any length of time I find that snails tend to have shorter lifespans and some leathers don't open up very well..
I haven't had any really sensitive corals or inverts so I can't say what any experiences were with nitrates.