About 50% of my business is lithography and I concur with Les. I will mix up a tank (5-10 gallons) of developer, then wouldn't you know I will get a void in lithography leaving the developer to sit for sometimes weeks or even a month without any real usage. Then in comes and order for 50 20 x24 plats. I find that so long as it is air tight during that time, the discoloring of the developer is insignificant. Since my tanks have to accomodate up to 50" wide, I accomplish the airtight by either pouring it into a 5 gallon collapsable bladder or overlaying the tank with sheet plastic depending on my need for tanks that day. I run five major printing processes but only have room for six tanks, so sometimes I have to pour off but usually not. Also it is interesting to note that old lith developer can be used to bring more contrast to multigrade b&w paper if/when using contrast filters isn't convenient or the contrast only need a bit of extra oomph. Anyway even with just the sheet plastic, it keeps nicely well into two months if not replenished and basically forever if it is.