Wow, I too am shocked at how many people would send their kiddos away for so long and especially with the distance. We also live about 10 hours from grandparents and there is NO way I would send my kids that far away for so long, regardless of how well I thought the grandparents would do, with the kids being so young.
I think I would consider it when they child is around 5. Little kids have no sense of time and I think they would think it was a very LONG time to not have mom and dad with them. I also know how stressful it is to stay at other people's homes when we go back to visit and I think that it would be even more stressful for littles to be out of their routine (no matter how well grandparents tried to keep a routine) and to not be in their own surroundings. It is one thing to visit relatives with mom and dad, but to not be left there for a week or TWO.
Regardless of what others say to do or not do, it really comes down to how YOU and DH feel about it.