N.H. Rev. Stat. Ann. § 44:13

In all elections by the voters in their wards the polls shall be open not less than 4 hours and may be opened not earlier than 6 o'clock in the forenoon of the day of election, nor later than 8 o'clock in the evening, as the city councils in said cities shall determine, at least 30 days prior to elections; and but one ballot shall be had during the day of each officer to be voted for.