Medical insurance needed for each visit on super visa?

[j.pallavi16]

Hi all,
When we applied for my parents' super visa, we obtained a year long medical insurance per the requirements. Their visa was approved and they already have visited me shortly after the approval using the insurance purchased.
My question is: do they need a year long insurance policy each time they visit me after this on a super visa? Or Is it enough to simply get insurance coverage for the short duration for which they will be visiting?

Thanks!