Minuteman Bike path from Arlington to Bedford, Narrow Gauge railbed to Billerica, Concord Road to Rt 3A to back roads to Lowell. Most days I ride one way and take the commuter rail to/from Medford/Lowell the other way. This makes it a 25 mile round trip.
I have been doing this commute for 2 months. My previous commute was Arlington to Woburn, 20 miles round trip.
It's more than 2 days a week in the summer and less in the winter. Yearly average might be 2 days / week.