You can't get visa for them for 5-6 months in one go .. first they will issue one month visa, then your in-laws will have to undergo medical exam, then it will be extended for maximum 2-3 months. I am doubtful that they will give extension for another 4 months after 1 month as they have recently rejected some cases.

However, be assured, if all the documents are okay, you will surely get it for atleast 3 months !!