Hey welcome to the forum. It depends on what your target is. If say you want about 4ppm PO4 every week, add this 3 times per week:
1.Measure 35g KH2PO4.
2.Add to measuring container
3.Add distilled/ro/rodi water until the 500ml mark is reached.
4.Shake well and shake some more after a little while
One dose of 10mL will add to your 300L (75g) aquarium ~ 1.3 mg/L PO4 and 0.4 mg/L K
I have 1 year old bottles that are still good. If you see precipitate that does not dissolve when the water is warm or fuzzy things the bottle is contaminated. This means that some PO4 was taken out of the solution and reacted with something or used by bacteria. Such bottle might still work but you get some indeterminate degree of variation.
To avoid bacterial growth you can boil the water and leave it to cool a little before mixing the solution. It also helps dissolve things faster. Add some Seachem Excel or Easy Carbo to the solution. Keeping then in cold dark place also helps.
On hiatus till later this year