Hello Miss

I have permanet residency valid till 2018. In 2014 my children came in spain having 1st residency valid till 2018. They want to go back in pakistan for their study. how much maximum time they can live in pakistan (every year or till 2018).

Is there any law they can live in pakistan for to complete their study and came back at expiry time of residency in 2018.

Thanks and awaiting.