I believe 250iu two times a week does the job for preventative measures. In fact I read a study that stated anything above that starts to not have much more of an affect than just 250. And yes it will really start to Degrade at about week 6.

The FDA currently requires doctors and hospitals to discard most things after 30 days of refrigeration, so I would say a month or possibly 40 days on the safe side. But I have seen someone say that after 60 days it has only lost 1% of its potency.
My philosophy is to always play it safe though because its your body you are messing with so don't take any shortcuts.
But 5 weeks fits very close too the 30 day timeline just mix it the day you plan to use it, I purchased a small mini fridge with a lock for under my desk they are relatively cheap at most large stores. So you don't have anyone messing with your materials.