Tenancy agreement without end date - legal status.

Dear All,

I have a commercial shop floor space which was rented out by me in 1999 by a written agreement. The problem is the agreement has a tenancy start date but it does not have an end date for the tenancy period. The total rent given is Rs 8000/- per month with 15% increment every three years.The property is located in Kolkata,West Bengal.

When will the tenancy come to an end as per the agreement ?

Kindly guide.