How long can I go without updating from 5.0.2

Hello xda forum. I currently have a samsung galaxy s6 that has been rooted. I actually tripped Knox in the process by using TWRP. and I was wondering when do I have to update before my apps stop working because of my android version being too low? I was reading online that Android 6.0 marshmallow was going to be released in the fall so I'm wondering if I'm going to have to update to that when it comes out. And how do I update when I'm rooted on 5.0.2 I can't use an OTA update because I tried going into the settings and I tried doing a software update in there but its not letting me so how am I supposed to update my phone to marshmallow 6.0? I'm from Canada and our cell phone plans last two years so if I can go without updating for two years I will be happy. can somebody please reply saying what I should do?

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