My first Sheltie (who ended up being a tiny little guy at 12 pounds) took 2.5 years to grow his coat. In all his 17 years, he didn't ever have an excessive coat. Considering my current sheltie, who at age 6 is a furball with feet, I had an easy time.
My current Sheltie, Bela, started her seasonal coat blow about a month ago, and for about two weeks, we went around constantly trying to spit out floating hair. I had just had shoulder surgery, so splurged in having her fully groomed. Well worth the effort - I usually get her done twice a year. It cut down on the shedding considerably, though I still got about a pound of hair off of her last weekend.