Depends on what you are doing and where you are located.
What? As Brian rightly said, you need to have a reasonable clean environment that is dust free - class 10,000 is usually MORE han enough. However, if you are coating CSP's or other miniature devices, medical or other precision STUFF, then you may have to go to Class 10,000 or even down to Class 1,000 - I have seen that for conformal coating in the space industry.
Where? In the UK, we have to satisfy HM Health & Safety Exec. (No doubt this applies elsewhere but I dont have the data) They ROUGHLY mandate that the area MUST be fitted with fume extraction and be exhausted > 20 times per hour. Lights should be behind "Georgian wired" glass rather than Ex'd' and all electrics should be hard-wired. Zone the room 1 = safest = around the coating machine and drying cupboards 1 meter from material. Zone 2 = > 1 meter away = OK for prep and handling, Zone 3 = > 3 meters away = OK for cleaning, masking, de-masking and inspection.
Hope this helps, Graham