You get a letter telling you the date they apply from and how long they apply for. That's after you've received the fine notice and paid the fine for whatever (alleged) offence occurred of course!
As far as I'm aware, you also get a letter to notify you when your points expire (provided you don't change address during the life of the points....I moved and didn't get the letter!).
But is there any way to find out between these two letters?
Say for example you have a company car, where company also pays insurance. You probably don't care how many points you have so long as it's under 12. So you may not be really keeping track of those letters. If you leave company and buy own car, you suddenly have to buy insurance and insurance company will be asking how many points you have.