Skittles has three pitches.
The first one is actually cute, its low pitch and is usually done in a melodic manner. I'd put this one as about the same as a cockatiels highest pitch. This one is very tolerable and is the one I don't confront him on.
The second one is higher and gets irritating rather quickly. Skittles will usually make this one when he wants my attention. This one often results in him being put back in his cage til he stops if he does it prolonged. This is also his most common one, but barely and it rarely results in a prolonged act. I just don't let up when it happens.
The third pitch is borderline intolerable. For someone with high tolerance of screeching, it gets intolerable after 10 seconds, while for most other people its intolerable right away. Skittles only makes this noise when he sees danger or something along those lines. Fortunately, this one is much less frequent than the others. I'd also say that while this one is nowhere near as loud as a siren, its also louder than a smoke detector. I'd put it somewhere between the two.
The biggest factor when it comes to this is based on the birds. Some birds are very noisy others rather quiet. But the pitch is the same. Skittles is on the quiet side but I'm convinced that's because he's free-flighted.