So how long was spirit gone away from the herd?

I think the non-aging calfs were sort of a blooper; I personally believe he was gone for around a year. If you pay attention to the seasons in the movie you will find that right before Spirit is taken it looks to be about mid-summer. Later when Spirit is with the Indians it changes to autumn. Then we he is taken to pull the train it is snowy and winter. However right after that when he is reunited with Little Creek it appears to be springish. When he and Rain return to the Cimarron all the trees are green and all is grown which leads me to think that about a year has passed.

We definitely see a lot of weather, but I personally can't see how it could have taken a year for the events to take place. I think mid or late summer to early spring makes more sense.

Where on earth are calves at in this movie any? I do not recall seeing much calves. Foals yes like spirit was in the beginning but no calves whatsofreakingever. Sheesh