We have some thoroughbreds that have been off the track for some time (not sure how long. I'm guessing 3+ years), that live just fine in the pasture. They have acclimated to being up north instead of in Arizona. They are pretty hardy and tolerate the different weather here pretty well.
My quarter horses are pretty easy keepers. I will agree with Kassierae.
It really depends on each specific horse. Just like with the horses my brother's fiance brought up. 1 of them was a pretty easy keeper and the other three were not. Of the two we have still here..both have turned into easy keepers now after almost a year.