To anwer the question, any is too much. As has previously been stated these engines were not built with power adders in mind, with the exception of some of the ones with better bottom ends.
There are a few on here that have installed kits, and unfortunately ended up with the engine going bang!!! Thats not a pleasant thought.
As I've stated before on here, nitrous can be run with success if all the necessary equipement is installed and used properly. Thats the bottom line, of course, along with having an engine that is in really good shape.
I would not go beyond a 100-125 shot, and especially if you have no experience using the stuff. Its better to start slow and work your way up and monitor the system and your car. a 150 shot may work temporarily but you are asking for a lot on a stock bottom end.
I've successfully run my car on a 75-up to and including a 125 shot for a very long time, since 1998/1999 with great success. But it takes work to monitor it and use it appropriately. Some have gained experience from me here on their systems and what it takes to properly utilize it.
I'll reiterate and say that if you beat on the car all the time on the juice, its going to beat back. You can count on that. 1 mistake with your set-up could be its last.
The bottom line is nitrous is a lot of fun and a lot of
"Bang for the Buck" when used correctly. If used incorrectly its a lot more bang for the buck and time and labor to get another and install too. There is no definitive answer to what you can run. Let your conscience and the experience on here be your guide.