Since the late 70s, but there were a few years in there where I didn't commute. I suppose it's been relatively consistent since 1984. I didn't bike commute when I worked in an industrial district where the only road was a narrow busy 2-lane full of huge trucks and I didn't commute much in 1998 after my knees were trashed in a car accident, but I'd say I've done it for about 30 years in total, all added up.