I think it varies from business to business. I guess you just have to keep building your list, asking them what they would like and then give them exactly that what they asked for. This way your list will grow and you have to keep serving your list creatively and keep them entertained.
Always have the option for your email list to unsubscribe from your newsletter and check your statistics. If too many people unsubscribe... think of another way to serve them.
With today's various ways of keeping track of all your online marketing it is easy to see if your email communication is working.
Sometimes if you are just building a fan list you can't really see a difference in your company. It is only when you offer a product or special service with your email list too see exactly what is happening.
Then you can check the stats and see for yourself at which point you can retire and spend your time on a tropical island sipping on a Pinacolada.
I think for this to happen you need a very large name list(like 1 000 000) and a lot of them should be aggressive buyers(people who really love your product) with lots of money.
Anyway... that's what I think. But I draw for a living... what do I know?