Let me preface this with the fact that I am not a current churchgoer. With that being said, I do not really consider the money given to the church as a "charity" donation. What I mean by that is - even though you go to church for "free" there are still salaries to be paid, lights to turn on, things to be replaced and remodeled, etc. Without the money from the plate, a church would literally close down and you would no longer be able to attend. With that in mind, I would say you could donate whatever you feel the church is worth to you, taking into consideration your other debts. I think the person who suggested that it should still be a percentage - ie, 1%, 5%, one day's salary, had a good idea. Just remember that if you are not paying anything, then someone else is paying your share, since you are using the service.
Note: I know that some church money is used for soup kitchens, etc. I am not meaning to offend anyone with my post, but a church is a business too.