Does the length of a prision sentence determine when a spouse has grounds for divorce?

Yes, the length of the sentence would be a consideration. For example, two husbands drew prison time for drunken driving. A husband going to jail for sixty days for a drunk driving conviction probably would not be grounds for a divorce; a husband going to prison for three years for vehicular homicide while intoxicated probably would be.

A short sentence for a lessor crime probably is not grounds; a long sentence for a felony probably is. Each state specifies how long the confinement must be for it to be considered adequate grounds.