The preparation of a timetable depends on your course workload and your efficiency of completing them.
The syllabus for 1st year in any B.Tech course has fundamental engineering subjects, which are based on the core science subjects taught from 9th to 12th standard in school. Hence, preparing for JEE Mains simultaneously should not be too difficult.
As an engineering student myself, I can say with complete confidence that steady and regular studying of subjects makes it very easy to score in the tests and exams. I would suggest dividing hours of your day between B. Tech and JEE subjects. Try to study related topics on the same day as it will make it easier to understand the concepts. Most of the concepts are inter-related, which is a huge advantage.
With proper time management and sufficient effort, I can assure you that you can excel in both your B. Tech tests as well as JEE Mains.