It's always been 14 days to serve a written Notice of Intended Prosecution if it's not practical to provide one verbally at the time for certain traffic offences. We always used to tell people and provide a written copy but obviously speed cameras can't do that.
In my limited experience of receiving the dreaded brown envelope, it's usually landing on the doormat within 3 or 4 days. Surprisingly they're not sent recorded delivery, so I don't know what would happen if you just denied receiving it at all.