I seem to recall reading somewhere that 1 hour each way travelling time is about the maximum that most people are willing to tolerate (of course there are exceptions).
I'm very fortunate that I have options for my commute. My usual commute is bike to Haymarket, train to Livingston North, 10 minute cycle to office. This is what I use the Brompton for. There is actually a minibus to the office (no useful public transport) so I can do without the bike, I just prefer to cycle, but it is nice to have the option especially in snow/ice.
My 'long commute' is 16 miles each way on a road bike. That's about 2h05 - 2h10 total cycling time (over the hour on the way in and around the hour for the return, depending on wind, mostly). I usually do this once a week throughout the year and sometimes twice. In the run up to the Snow Roads (and for other events in the past) I extended the homeward run to take up to 2 hours. I wouldn't want to do that every day - partly because I want to run and swim as well and don't have time (or energy) for everything, but also to do it regularly in winter I'd need a much more weather-proof bike than my current lightweight racer. I'm very nervous/cautious about doing the long comute in snow/ice.
I'm not sure what maximum distance I'd do every day. It would depend a bit on the route, as others have said. If I could be confident that it would be gritted I'd be happier than if it was something that was going to be an impassible ice rink every winter. Also it is nice to be able to commute (short commute) in office clothes - though there's a couple of men who also bike-train-bike who prefer to change when they get to the office.