First, I've a sneaking suspicion you may be using too much volume of chemicals, but I'm not positive -- it depends on your print size, the size of that first pouch of Dektol you bought, etc. As a point of comparison, I most often print on paper of up to 8x10 inches in trays that are slightly bigger than that (maybe 9x11 inches). I use 1 liter of solution in my trays. When I used Dektol, I'd pour it back into a bottle and re-use it for another session or two after the first one, with no noticeable problems. (I now use another developer that works with a replenisher more-or-less indefinitely.) If you used an entire gallon packet of Dektol, even at working strength, to develop a handful of 8x10 prints in your first session, then you definitely used more volume than you needed to, even aside from your dilution error. If your packet was smaller or if you started with much bigger prints, then that might not be true. You need just enough solution to cover your prints comfortably; you shouldn't be coming anywhere close to filling your trays to the brim. In fact, my standard of 1 liter is probably more than I need; I just use that much because it's a convenient size, given the bottles I use. I think that 750ml would probably be enough, and maybe even just 500ml. You can test using your own trays.
As to the longevity of fixer, that depends on the specific fixer. Some, such as "plain fixer" (sodium thiosulfate in water, with nothing else added) goes bad almost immediately. Most commercial powdered fixers ought to last at least a couple of months in stock solution. IIRC, Kodak prints recommended maximum keeping times on their packets, so if you're buying Kodak fixer, check what the package says.
FWIW, a lot of people prefer rapid fixers because they're more, well, rapid than conventional fixers. Commercial rapid fixers usually (always?) come in liquid concentrate form, and most of them are less expensive than non-rapid fixers. The drawback is that most or all of them have at least a hint of an ammonium odor, and this odor is rather strong for some of them. In concentrate form, rapid fixers typically last several months.