When using 2,4-D on our pasture, how long until we can release the horses back out? Some have told me it will not hurt horses, while others say to wait seven days.

2,4-D alone does not have a grazing restriction for horses. However, you need to read your actual product label to see if additional precautions exist because of the formulation. For example, lactating dairy cows are not to graze herbicide-applied fields for seven days, hay is not to be harvested for 14 to 30 days (depending on the product), and meat animals should be removed three days before slaughter.
If the label for that specific product does not have restrictions indicated for horses, it should be safe to graze without a time lapse. Always be sure to read the label carefully.