I can say that dry mouth, at least in my case, can improve after 2 years, even 5 and longer! My parotid on the left side is atrophied as seen on scan, but there is another on the right, although I was radiated bilaterally, the right was a lesser amount. Plus there are pairs of the two other major salivary glands on each side of the throat, plus hundreds of minor salivary glands in the mouth and throat. Lately, they're trying to avoid or limit the dose so they can recover, usually 26-30Gy, and volume to around 49%, if I recall correctly, to the parotid gland, which has more thin saliva, and reason for the thick mucus after treatment until it recovers or the others start producing more to compensate for the loss. The two others have a mixture of thick and thin, although more is thin.

IMRT was developed to help with the salivary gland dysfunction, so IMRT can can be dose sparing, and dose paint areas to be radiated to limit toxicities.

Btw, my first induction chemo, which I had alone, had just as worse side effects as radiation did, so chemo effects dry mouth too, but in my case, it may be difficult to give more of an exact timeline being I had more that one treatment, and medications, temperature/weather, foods, sleep habit, and medical medical conditions can effect dry mouth, especually st night from sleeping with the mouth open, and when the salivary glands basically shut down from what I understand. As Christine often says, correct what's within your control.