A Volt owners manual says two years. What does yours say?
I take that to mean that oil will last for at least two years under mild operating conditions. ( The Volt motor is controlled by computer only)
and to extrapolate on that, the Ford hybrids have 2 different recommendations:
Energi(Plug in hybrid) models call for 2 year/20k Mi, but Check the condition of the oil filter after one year, replace if rusty...
The Hybrid (no plug) Models call for 1 yr/ 10k mi. both Full Syn.
the Energi models have twice the battery, and can go 20 mi on just electricity.