I have recently moved into a rented property and it has a large grapevine out the back garden. I am unsure how to look after it or when the grapes will be ready. There are a few small grapes on it at the moment which are red & green.
They take some looking after as they need to be pruned in Spring and treated regularly. However, if they're nice sweet grapes will probably be worth the effort. Pruning isn't easy to explain so google to see a video.
Grapevines grow very vigorously. They are usually pruned back to within about 3 feet from the ground and when growth begins in the late spring they are restricted to a few sturdy shoots. These are then pruned above the flowers when they appear. If left unpruned you will have a one plant jungle with a few tiny grapes that are inedible.