My question is how many miles on a engine is too much for nitrous oxide?

Don't bother reading an engine's wear by the number of miles on it, just make sure the motor is in good shape. I have seen many high mileage motors in excellent shape, and some low mileage ones suffer catastrophic engine failure. If your going to nitrous your motor, it should be in good shape, that doesn't necessarily have anything to do with mileage.

So, I hate to answer you question with a question, but what shape is your motor in?

I do push my car at times but mostly only when I race. So I think its in decent shape.

well i would have to say it doesnt matter because if your planning on shooting NOS then one of two things will happin either way. You will blow a perfictly good engine and have to rebuild it anyway. Or you will ebuild the engine like new.. I dont like NOS and in my town to many kids shoot the crap in thier cars stock. I dont know of any that survived but hay these kids are dumb and shoot NOS all the time so take it as you will.

It's fine to use nitrous on a stock motor (in moderation) if you know what your doing, and if you don't, not even a built motor will survive it. Nitrous gets a bad reputation because so many idiots have burnt up their motors and blamed it on the gas rather than themselves.

I can't assess your motors condition over a computer, I will have to leave that to you. A compression test can give you a general idea as to the condition of your motor, but it isn't enough to declare your motor in good shape. Start with a compression test.