That really depends how far you want to push it. So far I'm close to fully upgraded house, lots of public works projects and spending money on main street. I would estimate so far in the game I've spent almost 10 million bells.
If you wanted to you could probably make 1 million bells a day by farming rare beetles on the Island. That would be about 3 nighttime trips depending on if you have rich town or not. That would be an annual income of 365,000,000 bells, good thing there is no income tax in Animal Crossing!
