You can check this website to get an indication of the points calculation:
For the points needed to get free ticket, it also depend on the fare and promotion. But one thing is quite good is that you can book a ticket using Big points only or a mixed between big points and payment. For example, let's say the ticket you want to book will cost 10,000 points. You can reduce the number of points to use and complete with a payment: 5,000 points + xx$$. For this you need to use the application "Air Asia Big loyalty" only. of course, you need to register with Air Asia.