If you want to study the Aerospace Engineering then you have to qualify an entrance. For BTech course in AE you need to apply for the entrances such as JEE MAINS & ADVANCED (if you want to study in IIT's/NIT's./IIEST/IIST), state level entrances, private college entrances. Now based on your choice, you have work and practice. Now in general there is no time limit which certifies that if you study this many hours you will surely get a chance. You have to do consistent and regular studies which requires a minimum of 4-5 hours study per day. Now if you are aiming big for JEE MAINS & JEE ADVANCED you have to study a lot and you have to work harder compared to that of the state entrances.

If you want to know about the colleges offering the course of BTech Aerospace Engineering & the admission process, you can consider the link provided below:

https://engineering.careers360.com/colleges/list-of-be-btech-in-aerospace-engineering-colleges-in-india

For more information regarding the career in Aerospace Engineering you can consider the link below:

https://www.careers360.com/courses/b-tech-in-aerospace-engineering

I hope this helps. Wish you a great eve!