PBS can be used as long as it doesnt have any visible contamination. 25 days is not a long time for PBS. Dont worry about that. If you have doubts, just re-check the pH.

The possible storage time depends on the chemical, of course. I dont know which other chemicals/solutions are you talking about. For instance, many substrate solutions are freshly prepared before using them in ELISA, unless they are commercial ready-to-use solutions.

Which concentration of BSA is the best for ELISA experiment ?

I have seen people using it in  a range between 1% and 5%. When I use BSA as blocking agent, I normally pprepare it at 4% in PBS. But most of the times, I prefer to use 4% skim powder milk instead.

very good response by Rojas. Details of Guidelines and protocols, see it

https://tools.thermofisher.com/content/sfs/brochures/TR0065-ELISA-guide.pdf