I would imagine you'd get the best out of it when fresh. However it isn't completely reliant on the enzymatic action seems as it's <1% enzyme... so unless you've got the grottiest, mankyest indian restaurant to clean then a stock solution several days old would work fine I'd think.
45 scoops, must take all day! Get a bigger scoop!!

Seriously though, the smaller the scoop the bigger the room for error when measuring out a lot in one go. Both shockwave and SPM weigh the same in grams as their volume in ml so you can use a bigger measuring cup. Saves a lot of time and will be much more accurate. I've got one of those large 50ml measure scoops too which is handy, just a larger version of the one that comes with powerburst etc..
P.S. When I've used it on mingers I've also add a healthy dose of oxidiser into the mix as well, seems to work very well.