Hi Starter, welcome to TP
Sorry but there is no time scale, my wife had to stop work at 50 and was diagnosed at 55, 12 years later she's still going strong, granted but she has no other physical issues. One member here was diagnosed 16 years ago (I think) and still posts on here very coherently and makes some very interesting and profound comments as do many of those diagnosed.
Sometimes the decline may be rapid but often it's a slow progression it's more likely other health issues may come into play before AZ is the biggest danger to someone's health, in a physical sense.
The most important thing is probably to look at putting all the pieces of the jigsaw in place now for what may happen in the future, POA, wills, housing and all the other bits.
I had to come to terms with it some years ago with my (now late) mother now I'm doing it second time round with my wife and there is a lot to learn.
Practical side over "Just feeling frightened really " don't be, we're all here to help as much as we can
K