How much is reasonable for a hobbyist to spend on camera gear?

It really depends on your disposable income so there is no set number but here are my words of advice:

First of all, if you are not married but planning to be, keep separate bank accounts after you get married. You will have far less arguments about money.

Second, spend only what you will not miss. If your house and cars are paid for and you have no credit card debt then buy what you really want and don't look back. Otherwise, spend your found money on these things. If you get a bonus or sell something, then this money was not planned to pay bills, so enjoy it. Don't go in debt for gear unless you will make money from it.

Finally, always take lots of great pictures of the wife (and kids) with the new gear. This will help to show them the value of it.