If you are using distilled or boiled water and storing it in an airtight container (like for instance a spray bottle) as long as it is straw in color then it is still active...My spray bottle stays straw color for a couple months.
A few of us who store larger quanities, rather than dump out the solution, just "re-prime" it with some fresh iodophor, until it goes back to the straw color.
I use bulk iodine dairy sanitizer that I buy for about 19.95/gallon from Tractor Supply COmpany...The only big difference is that it doesn't have an anti-foaming agent added like BTF's so it will foam up a bit (not as much as starsan, but still some foam) If I don't want to wait for the foam to subside, then I add a drop or 2 of fermcap foam control to the solution.
Unfortunately, in some states you can no longer buy it, because evidently it's also popular to make meth with....so people will have to check at their own localities, and their own farm and feed stores.