In many ways it is very subjective. Subjective in the sense that the amount of treatment follows the intended use for the listening environment. One can go for as little as 2 panels if the existing space is already pretty dead. e.g. heavy drapes, heavy carpeting, lots of plush furniture, drywall. In high rise condominiums typically finished with CHB and poured concrete all around upwards of 30% surface coverage is not unheard of. In other words if one is trying to do Mickey Hart's drum kit at home, get those panels and bass traps ready. If it's Julie London perhaps not anywhere near as many.
Ultimately what at least I shoot for is neutral vocal presentation or what I refer to as perceived flat in the midband. If the voice becomes chesty then there probably is too much high frequency absorption. We all know what the voices of friends and relatives sound like so the test is if after treatment I can speak softly with someone I know across the room while being able to hear what the other person is saying very clearly. Naturally he has to sound like himself as I would hear his voice in a wide open space and not get morphed into Barry White. That's the first and most important hurdle. This simple test shows that mids and highs attenuation is fairly even, the lack of smearing of consonant sounds shows that reverberation has been decreased and the noise floor has dropped as well in the process. Small ensemble recordings with little bass should improve greatly at this point. As bass output increases, then things get more complicated. As nodes become excited the room can be acoustically overloaded. In extreme cases of overload, bass in the room actually diminishes only for it to boom in an adjacent room or hallway. The accumulated pressure seeks an exit in effect turning that hallway into a horn. The addition of bass trapping becomes mandatory for systems with high output low end capability in rooms where the longest dimensions are less than half of the wavelength of the lowest frequency that the system is capable of. One can look at a room as the inside of a speaker cabinet. Just like the inside of the speaker cabinet excess energy needs to be damped lest the cabinet itself resonate. Depending on many factors including building materials and construction room resonance can actually begin at what might seem benign levels. The common approach is to convert that excess energy into heat by way of the little fibers in the treatments that slows down the excited air molecules through friction. The second most common technique is to break up the pressure wave like quadratic diffusers do where force is distributed over a wider area and cancellations occur at the areas of deflection inside and around the cavity mouths. Leaving only a portion reflected directed back to the listening position.
I believe the question is however how can one tell if he's gone overboard. Well for one, if speaking voice or music no longer sounds natural it's time to take some down little by little. If your 30Hz capable floorstander starts sounding like a two way monitor then perhaps there might be too many bass traps.
So all things being equal, in the end it is up to the individual to decide how much reflected sound he wants in his room along with the direct sound. Being a purely personal decision. IMO there is no better method than trial and error the first time out or contracting a professional whichever comes cheaper