How often to flush/change dot 4?

My Duster is going through a major rebuild, engine, trans, exhaust system etc. It has been sitting around for 2 years now. I plan on swapping out all four wheel cylinders and the master. May as well flush and change out the fluid if it needs it. It had all new fluid 15 years ago, but I don't know how long the brake fluid (dot 4) lasts seeing how it can gather moisture.

Any input? Thanks.
Drain the master almost completely....then fill it cap it and bleed it almost empty...then repeat 3 times.