I always mix my own D-76. I can scale the amount to what I need. It takes less than 10 minutes to mix up to a liter of stock, and not much longer for larger batches. Most of the time is spent dissolving the metol, with the sulfite a close second. The stock solution keeps decently well, but I usually just mix what I am going to use that day. You don't need to use hot water to mix up a 1+1 diluted solution, so you can use it right away. (However, some people believe that mixing up stock and letting it stand for a day gives more consistent results.)