You can only get 99,999 bells maximum in interest at one time. If you travel 1 year, you'll probably get 99,999. If you travel 20 years, you'll still get 99,999 because the game will only give you one set of interest. I wish it stacked, but it doesn't.
Edit: I could be wrong judging by the above post? This is the way it was in Wild World though!