Hello,
Thank you for posting your query.
Yes, it is a legal requirement to get the rent agreement registered, since unregistered documents cannot be used in evidence effectively.
Because the term of tenancy is for 33 months, hence the flat owner can ask you to vacate the flat after completion of the said period. However, in order to get the premises vacated (in case you are unwilling to do so after he gives you notice for the same), the flat owner needs to file a case before the court of competent jurisdiction.
You mentioned that you are paying the rent in cash and not getting any receipts for it. This may pose a problem for you to prove to prove that you had been regular in paying the rent before the court of law.
Regards,
Nishant