The longest commuter I ever had was 17km each way, but the route, while mostly protected, was not nice or exciting and seemed like a slog, it was a lot to do 3x a week (I'm mid-40s, that does matter) and left me tired for the weekend rides. Currently doing just about 7-8km each way, shortest I've ever had but it's over the exceptionally unpleasant Knight Street Bridge so it is my least preferred of all.
Best commute I had was 25km round trip, did that 3x a week year round, the last part was along the river in south burnaby and I had a selection of several routes to get there so it was overall my favourite. Perfect length, really nice ride, I could have made a calendar with all the beautiful pics I got along water.
I've been job hunting in the last few years and I absolutely consider the bike commute to a new location, it's a big quality of life thing for me. Heavy wildfire smoke is the only thing that has kept me from riding, I have a set of wheels with studded tires for the winter which make icy conditions comfortable. My MTB stamina definitely suffers if I don't do the mid-week commutes.