17 miles for me each way, either down the delightful A6 into Manchester or following the canal towpath through Marple, bit of jinking through roads and estates then more off-road through Reddish Vale Park. Finish with either tarmac cycle path or a few miles of city roads.
Off road is great in good weather but if it's wet the canal towpaths just turn to horrible gritty mush. On road takes about 1hr (SS road bike), off-road takes about 1.5hrs (CX bike) although I did it a few times in the snow on my MTB and it was getting on for 2hrs each way then.
It's too much to do the off-road route both ways every day. Sometimes I road ride in then off-road home, occasionally I'll get the train in and ride home, if the weather really is vile I'll usually drive.