The life of liquid concentrates and mixed stock solutions from powders isn't really that different, depending on the specific solution. Liquids are a bit more convenient. Fixer doesn't last forever, and all chemicals get exhausted with use. For the occasional darkroom user, it is probably best to through the working solutions out after one session, unless you are going to do more work in just a few hours. Stock solutions of developers like Dektol last about 3 months, depending on conditions. Film developers, like D-76 and Xtol, may last a bit less time. There are a few liquid concentrate film developers. Most of them have an open bottle shelf life of a few months, although HC-110 is noted for its longevity. You can get several liquid concentrate print developers, like LPD and the Ilford developers, that have about the same or slightly longer life. I don't know where you are, but in the US you can get Liquidol, an outstanding print developer with a significantly longer shelf life (and tray life) than the others. Liquid fixer concentrates have a very long (but not indefinite) life in the bottle. Fixers mixed from powders do not seem to last as long. As you become more experienced, you may want to invest in a good scale and the chemical components to mix your own solutions.