We have kept SH 12.5 in original dark blue or black 55 gal drums for up to 4 weeks with no noticeable difference. Keep inside, no direct sun, cool as possible. check with your supplier for any other ideas they want your sale and know their product...Also ask minimum percentage your supplier guarantees...
Usually 10% up to 12.5 as industrial launders need guarantees for washing programs.
Large suppliers get train cars of SH and break it down to totes and drums etc...
hope this helps
Chris

I still buy it in 5 gallons. get 25 30 gallons at a time. I mix on site still. Pain in the arse till I get a 15 gal horizontal tank for the trailer. Just depends on how busy you are, I AM, but still using old methods from before. Dont fix what aint broken... I'll be changing some things soon.

It must be cooler where you guys are at.  In Houston TX I store mine in an open covered area.  I see a noticeabale difference after 2 weeks.  After 3 weeks it is salt water.