Mine is 17.5 miles each way, it's a fraction shorter if I do the off-road version (although that takes at least 20 mins longer) and there's various different ways I can combine the options open to me - half on and half off road is a viable route.
It's a fraction too far to do every day though - I have done it 5 days a week in the past and it just leaves me wrecked over the weekend, I don't want to look at a bike!
When I worked 12hr shifts I had a nice 8 mile rie which was about as far as I'd want to do on that shift pattern.
I've also done occasional days working at a site 24 miles away and that was a beautiful commute along country lanes, hardly any traffic and in good weather it was stunning - agian far too far to do every day but on the odd day a week I worked at that site I used to really look forward to the commute, I'd be upset if the weather was too crap to ride.
As for the OP question, I'd say about 12-15 miles is perfect for an every day ride - much more than that and it starts to wear you down. Being able to vary the route makes it a lot more bearable.