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