Ans: If you're a tenant - as long as possible! Why?
1) To save on registration + broker costs every year.
2) Some landlords use 11 months because no legal requirement to register it if tenure<12 months.
But SOME States require registration anyway (even below 12 months). eg: Maharashtra.
3) You ideally need a registered agreement to claim
HRA.
4) In case of disputes, a registered agreement is better protection than unregistered.
So bargain for long term agreements!