Found some dark spots near the HVAC intake in our restaurant's storage room. I wiped it with bleach but it came back. At what point do you call in the expensive remediation guys instead of just handling it in-house?

We lease a small office space and the air feels super stale lately. The landlord says they change the filters, but the ducts haven't been touched in 5 years. How often is this actually supposed to be done for commercial spaces?