I haven't had a chance to observe homozygous/heterozygous grays to much. I can tell you from personal experience though.
BLACK Gray Mare (MFT I think) - Started to show noticeable lighting in the face and body by five. She got progressively lighter, but 'gray gray' didn't start showing up until about five years old.
BLACK Gray Colt - Son to the mare. Noticeable graying by two years, born true black.
BLACK AND WHITE TOBIANO Gray Colt - Son of the black colt, by a black Tobiano filly. Noticeable graying by one year old, he pretty much showed real graying after he shed his baby coat. Born black and white Tobiano.
September '10. He turns two in March I believe.
Unfortunately, no baby pictures :( I have a few pics of his dam, but none of his sire or granddam. I think his grandsire is Dusty, sire of my Loki and Redman. Not sure though, I'll have to check when I pick him up. He's been gelded, so theres no telling how fast his offspring would gray out
This is the first I've heard, I don't own any grays and haven't known any since they were young, but I know it varies by horse, by melanin production, I'd be interested to see if the Gray gene is in fact an incomplete dominant
The grey gene can't be incomplete dominant because the phenotype of the heterozygote is indistinguishable from the dominant homozygote. If grey were an incomplete dominant then it wouldn't fully mask the base color. It would simple result in something of a lighter color, similar to the cream gene. With snap dragon flowers you can have red, white, or pink. The pink is merely a result of incomplete dominance of red over white (I believe it's red over white, could be the other way around). I've never seen a marked difference in a homozygous grey and a heterozygous grey when it comes to the speed of greying out. I wish I had some pictures of one of the Thoroughbreds I know. I do have some of two other greys I know though.
7 y/o (I believe, she may be 6 going on 7) Arab/Hanoverian mare.
To be honest, there has to be more at work than just the grey gene. Some homozygous greys do grey very quickly, but then so do some hetero greys. I hope they figure this all out before I die, otherwise I will be wandering around the afterlife with lots of questions lol.