Every 2 years might be too long. The DOT 3 fluid has agents to absorb moisture and indicate they have done so by turning darker. I wouldn't let mine get past a dark honey color before bleeding out the entire system. The alternative is moisture can build up and cause rust in the system, eventually causing mechanical failure. This also is the cause of many spongy brakes even when you've got the air out of the line. water has a different compression rate than brake fluid. If you've never done this and it looks as dark as your morning brew, it's way past time. Dot 3 actually draws moisture in(and it disperses evenly throughout the system).
You can switch to dot 5(silicone) but it does not draw moisture in. It will actually allow moisture to collect in the lowest spot. If there is no moisture in the system to begin with, this is never an issue. If there is moisture and your brake cylinder vaporises the water boiling at 212(adjusted for pressure) you can theoretically have a cylinder lock-up. Personally I prefer to clean out the entire system(off the vehicle bore cylinders, etc) and go back in with DOT 5. But I'm well aware of proper care.
There's also a new DOT 5.1 (Not silicone but similar to the DOT 3 that's supposed to be better) but I've not had the time to research it yet.
Hopefully that doesn't confuse you. Learn a bit more about them and you'll feel safer making any changes, or keeping what you've got.
1977 holiday rambler