From a PM point of view I think that at least 12 months is ideal.
A 6 month lease is harder to attract new tenants, it's such a short time to move in, get settled and have the uncertainty of whether or not they can stay. Having said that, I often do initial 6 month leases with the view to renew if we have a tenant moving in without much in the way of rental history (I don't charge my landlords to relet to the same tenant!).
My concern is more with when their property could potentially fall vacant. I try to avoid leases ending in December with January being the better option in our area. 13 month leases are fairly common place.
Some properties are harder to lease in Winter, with this in mind I try to arrange longer or shorter leases on specific properties that I know will be hard to lease if the current lease expires in winter.
A less than ideal tenant is never easy to get out but with the first routine inspection taking place at 3 months (in VIC) then I always have time to issue a 90 day notice to vacate if required (end of fixed term tenancy notice in VIC). Often the tenants that end up being awful stay well behaved while on the lease and only fall into issues when they are on the periodic (which we can't stop happening here).