I will *only* master batch my lye now. It has saved me an unbelievable amount of time when it comes to making soap, and while it isn't difficult, I do recommend waiting until you're a little more comfortable with making it the usual way (if you're new to soapmaking). My entire reason for starting to master batch my lye was to help eliminate soda ash. I live in a fairly arid climate, and found that regardless of what I did, I ended up with atrocious soda ash on my soaps. I read somewhere that water discounting could help with that, and researching *that* led me to master batching my lye. LOL I now use a 33% solution, and have had either very little or no soda ash.
I make 30 oz master batches at a time, and I store the COOLED lye pre-mix in these containers:
32 oz Utility Jugs. I've had absolutely no issue storing it in these. I do NOT mix it in these; I have plastic HDPE buckets that I mix in & allow to cool to room temp (without issue), then funnel-transfer to these. I make soap about every other week, and have had zero issues with the longevity of the lye solution. DO make sure to shake it up before you measure out what you need. (Well, gently shake it up. I pretty much just tilt the jug back and forth a few times.)
As far as heating it up goes, it's going to do that automatically when you add the required amount of extra water to it, per the calculations, so I heat my oils first, and when they're down to about 130 degrees, I go ahead and add the extra water to my lye pre-mix solution to bring the temperatures in line. I soap at ~110 degrees.
Hope this helps!
-Maevyn