Depends on how the location works for you. I've usually tried 6 months first then if it ticks all the boxes for eg no nosey parkers, and good landlord relations eg repairs and stuff may try a year next time.You might renew for many years if it suits you. Why move every 6 months unless there's a problem.
I agree with salty - I've just renewed my tenancy on my work flat for another 12 months and I've no plans to go anywhere, but it has a 6 month break clause so if I wasn't happy (or the landlord wasn't) I could have moved on then. I don't know how common they are, but it's worth looking out for .
A lot of 12 month contracts put the 6 month clause in for both parties benefit it's quite common but if one landlord did not you could always ask for it.