Ive got a greenhouse on my plot and cant get to it every day during the summer. Sometimes every otherday, but if working away can be 4 days without water
It has an autovent on the roof, and a vent in the back wall (left open in the summer) also if 'red-hot' ::) I just leave the door open.
I grow toms, peppers, chillis and cucumbers in it - with a little thought you can reduce the ammount of watering needed....
1) Grow veg in borders - or in growbags with minimal drainage holes and water retaining gel in them
2) If possible rig up your down pipes to run into the greenhouse and water when it rains
3) Shade in hight of summer
4) Reduce water evaperation by covering exposed soil - black plastic over borders, or newspaper over growbags
or... if you have a neighbour who wants a greenhouse, but doesnt have one offer them space in yours for a few toms etc, in return for watering your plants when they do their own? ;)
Cant speak about Poly tunnels.... personally I would go for the greenhouse... ;D
Whichever you get though you'll wonder how you ever coped before