TOEFL: is there an average number of attempts?

[ScorpioPower]

I'm based in Thailand and have some fairly smart students. They have never lived outside of the country before.

I'm using Longman TOEFL.

This is the first attempt for the students and it is very unlikely that they are going to be successful on this occassion.

I'd like to be able to tell them about how many attempts other students make before actually passing the exam.

Has anyone experienced an average number of attempts?

Please let me know!

Cheers.