My typical day is a total of 12 miles, split into 4 roughly equal segments. The longest I've done in one spell is about 6-7 miles across London. Journey should have been more like 5, but I got kinda lost.
Bear in mind that the bike is a single speed Halfords "Apollo" folder, so it isn't necessarily that comfortable.
What felt like the longest journey was last thursday night. As a result of slight weather issues around London, I must have done a couple of miles pushing/carrying the bike, just around waterloo attempting to get onto a train.