Decanting will expose it to more air so it will oxodize faster. Depends on the wine I guess. I would say drink it in less than 3 days.
I would say only decant what you intend to use that evening.
The undecanted wine in the recorked bottle will still be oxidizing. I do not like to go much past 3 days but some say red will last 4-5.
A lot of what I told you is opinion that may contradict your own as you experience more, hope it helps, but keep in mind it is very subjective.