Very much dependant on species and time of year...
Shortest: a 2 hour winter chub trip, sometimes half an hour of that is spent walking the bank and feeding swims, before settling somewhere good for the witching hour. I will often go home as soon as I've caught one (they tend to be big on the thames)
Middle: a 10 or 12 hour day on a lake - tench, carp, a bit of everything. Me and 3 mates have a couple of trips a year where we'll fish somewhere with attached accommodation for 3 days, no night fishing involved but it's certainly easier to get up and go knowing that your gear is already set up and by the water!
Mental: 18 hours or so - sunrise to sunset - on the Wye in mid-summer, barbel and chub in mind. There's just nothing to beat that feeling of tramping all the river during the dawn's first light, knowing that you'll be on the river until 10pm or later. The beginning and end of the day tend to be most productive, though I've seen enough barbel caught at 1pm on a sunny day to make a mockery of this. Can be a bit gruelling, but it's a very short season as once the school holidays arrive, so do flotillas of canoes and you're back to fishing mornings and evenings...