The northern route, Grand Isle to Plattsburgh, only takes 12 minutes and runs 4-5 departures each hour during the day and at least 2 departures all night, everyday, all year. The docks are a bit north of Plattsburgh and Burlington.
The middle route is between Burlington - Port Kent takes an hour, operates in the summer and fall with about one departure per hour. This is the senic route. I'm told it puts you within a walk of the center of Burlington. It is closed in the winter.
The southern route between Essex (just north of Westport) and Charlotte (south of Burlington) takes 20 minutes and operates all year, less frequently in winter. It operates during daylight hours only. In the summer, from 7am-9:30pm or so, there are departures every 30 minutes. Check the website for spring, winter and fall departure time which are more like 1 per hour, but it does run all year.
Current there is a ferry operating 24hrs 365 days a year at the site of the Crown Point Bridge which is being replaced. This is free.