The letter is short, written on A5 paper (half sheet). I assume that everyone who wrote to Camilla, to wish her good health, received the same typed letter. The typed portion doesn't appear to be personalized and only mentions vaguely what I wrote about. Camilla personalized the letter by writing the persons name and hand-signing it.
I sent my letter 24 days ago. It was responded to quite promptly.
If you are still waiting for replies from the Cambridges for various events this summer. Those would be handled by the General Correspondence section at Clarence House. They are behind on responses. When will they come? I don't know. To be honest I expected to see responses for at least Prince William's birthday a while ago. But they have been sending out responses, so don't worry, they are just still Pre-William's birthday (Early June).
When Camilla hand-signs letters or cards, my guess is those are handled personally by her office, I.e. a assistant Private Secretary. Which is why we see much quicker response times.