After 2 years commuting, I don't even think about it now, I just cycle to work every day. 11.5 miles/45 mins. It's definitely down to time rather than the distance. I don't mind getting up 20 mins earlier than if I used the motorbike.
The return journey takes 35 mins if I'm feeling eager, only 15 mins longer than I used to do on the motorbike (heavy traffic), so really I'm only losing 15 mins with the kids in the evening.
I reckon my limit would be if it took say, 1 hr 15 mins mark to get home. If it did, I don't think I'd be cycling 5 days out of 5 any more, 'cos I'd not see the kids enough before bed time.
I considered doing it every day, but with each stretch taking around 1h 20m, and it having to be the same route every time, I quickly realised it would just be really boring really quick, to the extent it might just kill my love of cycling.
I noticed that after around 6 months of cycling to work, I was setting off, then just appearing at work....I've never felt 'bored' doing the same route every day. I still find it relaxing having 45 mins each way where I can just chill/daydream, or listen to an educational podcast.