Justin, the most instructive test I have found to determine the State of the Soup is to simply drop in a test scrap with the lights on. For one, it will tell you if the developer is active. Two, it will show if there is enough semiquinone present (in form of old brown) for infectious development. Three, it will demonstrate need for a restrainer, i.e. KBr or NaCl. (A dash of benzotriazole will also work.)
As I do quite the same as you, printing over the course of a few days, I have found the best way to preserve the activity of the developer is NOT to pour it into a jug for later use. For a small tray, cover it with saran wrap or like plastic wrap, and for large trays a kitchen garbage bag sliced in half will work well.
Press the plastic into contact with solution, and it also seals well along the edges. Work the little bubbles to the sides first.
Oxygen is the enemy. Eliminating surface area eliminates oxygen. Pouring a solution back into a jug (and back into the tray again later) creates turbulence and increases the surface area by untold amounts, albeit for a short time. Perhaps this goes against convention, but when you pour a solution into a jug you have effectively mixed in a giant dose of air; put a cork in it and let it cool, and the O2 has nothing else to do but oxidize your developer. Of course, if you can't leave your tray out, that is a different story.
I don't know the ballpark lifespan, Justin. Throw in a scrap at the start of your print session, and you will have your answer.
