The Volt OLM allows up to 2 years and no mileage limit, while other GM cars are limited to 1 year or 7500 miles, whichever comes first. Since the Volt is a PHEV "extended range EV" you can probably guess why GM allows a longer max interval in it than other cars they make. The hard limits started about 10 years ago when dexos was first introduced. Before that, GM's OLM systems never had any hard limit, but suggested you not exceed 12k or a year.
So for your Volt, change the oil when the light comes on, or if it doesn't come on after 2 years, change it then