Saturn 1997 SL-1. My brakes work OK but don't feel especially strong. I've had them looked at - the dealer changed the brake fluid and did some adjustments on them. For planning purposes (financial), I'm just wondering what is a general rule of thumb on when brakes need replacing - or if there even is a general rule of thumb. In other words, after 50,000 miles, 75,000 miles or 100,000, etc.?

This is something you will have to chart for yourself. Mileage between brake jobs will vary depending on the type of driving. More highway higher mileage. More city lower mileage.
I rotate my tires when I change oil. While the wheels are off I check the brakes. After keeping records of your mileage and brake jobs you will have a pretty good guide for you and your saturn.