Got flashed by roadside camera today. All I want to know is how long do they have to send me a letter?

I believe there is a legal time frame in which they have to notify you otherwise they can legally enforce the summons/fine etc on you... is that correct?

If so, what is the time limit?
If i recieve the leter after this time has lapsed, is there a standard response I should send?

I hate the waiting period involved. I can deal with the outcome as I'll get what the law sees fit.... unless it's a ban, need my car for work (38 miles each way)

Thanks