I'm sure his giving 25 years of salary back to his church and 90% of his earnings from his book is far more than you or I would ever likely give in all honesty. Besides God doesn't look at the amount a person gives but in what spirit it is given.
The last part is true and the first part is unknowable. I'm not saying he is wrong in giving. I'm just turned off by his announcing of it. Wouldn't God look better on it if it wasn't plastered all over the news as proof of his devotion? Besides, I think you can be a very charitable person and still not a christian or a preacher of the real gospel. So my point is that that is nowhere near enough. He draws people with an appearance of holiness but he preaches what looks to me like a different gospel than the gospel of Jesus. Jesus started out with the call to repentance.
Further..his giving to his own church is sort of like investing the money back in his own business. He benefits.