Family visitor visa for 2 years

Hello,

I am applying visitor visa for my father in law for 2 years.What reason we should give in covering letter why we are applying for 2 years and not for 6 months.Any pointers will be appreciated.Thanks

Has he visited the UK before as a visitor? If yes, how many times has he visited and how long did he stay?

What is your reason for applying for a two year visitor visa??

Yes. He visited twice before this. Once stayed 3 months and once 5 month. Both times went before time.
This time also he is planning to go back in 2 months.

He is applying independently. So he needs to arrange documents again and arranging appointment again. Basically to avoid less hasssel of applying again.