Google no longer supports time ranges for cook, prep and total times within recipe schema markup and structured data help document. Google said it removed the "guidance about specifying a range for the cookTime and prepTime properties in the Recipe documentation." It should also list totalTime, because that was added as well.
Google said "Currently, the only supported method is an exact time; time ranges aren't supported. If you're currently specifying a time range and you'd like Google to better understand your time values for cook time and prep time, we recommend updating that value in your structured data to a single value (for example, "cookTime": "PT30M")."
So if you had ranges, like 10-12 minutes for cook time or other ranges, you will need to change that to a specific time and not a range. It will have to be a single time, not a range of time, so a single value for a time. This is unusual because most recipes have ranges, I can be making that up, the only recipes I've read are on the back of a Duncan Hines box.
The old help docs said "you can use min and max as child elements to specify a range of time" for cooke time, prep time and total time - that has been removed. Here is a screenshot where it said range was okay for one of those fields that was changed:
Forum discussion at Twitter.
