When you were at school you were probably told to decant a little of your buffer solution to use for each calibration into a little pot, then to chuck it away afterwards. This was good advice then as it is now. Your freshly made buffer will keep fine for a good while in a cool, dark place, just don't contaminate it by putting used solution back in the bottle. Buffers for pH values of around 7 are typically phosphate based so could probably support bacterial or algal growth in the right circumstances.