There is one ferry a day from Stresa/Intra. This is the 11.30 (from Stresa), which arrives at 13.20. The return ferry is at 17.20, which does leave a good afternoon to spend here after your lunch. As a bonus there is a hydrofoil on a Wednesday (both directions).
The Stresa service only runs on certain days in April/May/October and not all in winter. This is a disappointing service compared to Lake Garda's frequent ferries. For north of Intra, the ferries are much better from the Locarno end of the lake - of course if you stay in Cannobio you get a better service as most people on Lago Maggiore are heading for the Stresa area.
If you are going from Stresa you can also go on a half day coach excursion to Ascona and Cannobio. I haven't been on this but you only get an hour in beautiful Cannobio.
Take the boat!
It's much more convenient to arrive by boat than by your own car. Believe me italia police force do fine your cars if parked in a none properly mannour.
And any way, it's more fun to take a boat trip.
And why not stop at the Isola di Brissago, now that is must.