It takes about 1.5 to 2 years to write a book. Assuming you have a publisher interested in your book, the book would need to sell between 10-12,000 copies in order for the publisher to break even. 20,000 copies is considered a good seller. An established author would need to sell 50,000-250,000 in order to break even depending on the advance, printing, marketing budget, etc.

The amount of money brought in does not usually justify the amount of time spent on writing the book. There are very few full-time Christian writers. It's very rare. Most people supplement their income with their writing.