I prefer 12 months after an initial 6 month term, but tenants seem to invariably want to get out of them before the end of the term (!!) We personally seem to have had tenants staying longer when they are on 6 months terms, the renewal seems to come around quickly and they sign off on it. That said, however, I always make sure that leases do not expire in December or January - Dec is a very poor time to find tenants and we are generally overseas in January.