As per the information available, the BDS course needs to be completed with in the 8 years of enrollment. So if you are with in the range of 8 years starting from the year at which you get enrolled for BDS, then you are eligible to complete the 1st year exam backlogs. For example, if you are enrolled in BDS on 2015 then you can complete your backlogs with in 2023.

Although, I would recommend you to contact your college if there are any other formalities available through which you can clear your backlogs.

To know more about BDS, visit the link below:

https://www.careers360.com/courses/bds-bachelor-of-dental-surgery

I hope this answer helps. All the very best for your future endeavors!