There was a time where 12 month contracts were the norm, now they're a rarity. The most common option seems to now be 18 month contracts and some ISPs have even started to offer 24 month contracts like BT and Vodafone. That's good in some ways, tying you into great pricing for a long time.
But at the same time, you obviously have less flexibility, being tied in for an extra half a year longer. If you hate the idea of such a long contract, then head back up to our comparison chart and use the filter to see only year-long deals. There will be fewer to choose from, but there are still excellent prices to be had.
Or if you want something even more flexible, 4G home broadband is a great solution.