In my experience, the build heads don't need replacement unless they've been damaged. Damage to the kinematic coupling, such that it doesn't seat properly on the build arm, would require replacement, as would damage to the printing surface of the build plate. If your build plate has surface irregularities caused by the scraper during the part removal process, you can restore its smooth surface using sandpaper.
If you're using multiple resins, another issue to be aware of is that resin ingress into the build head can cross-contaminate the resin in your tray. Therefore, the best practice is to use a dedicated build head for each resin that you use.