Hi Daniel,
The best way to know that it's time to change the brush pen is when it begins to fray. That might happen if you're using a rougher paper to write on, or if you're pressing the nib too hard. Try HP 32# laserjet paper. It works great for both brush and pointed pen calligraphy.
Also, if you're new to the brush pen world I wouldn't recommend the Fude 8. Its pretty flexible, and most beginners have a tough time getting a hang of them. Try the Tombow Fude of Pentel Sign if you're a complete beginner, or a Kuretake Zig Brushables if an intermediate.