Qur'an

Ma'zur reciting Qur'an with the same wudhu of that particular Salaah time

Q: I have irregular menses for the past year. Sometimes whole month and sometimes less than fifteen days between. I have tried to calculate my menses and istihaaza days from memory but I am becoming unsure. Also I perform wudu before every salaah during istihaada but what if I want to engage in quran reading after each salaah? Can I use the same wudu? Also If I engage in quran reading after Asr and then Magrib time arrives must I make new wudu or is the wudu of Asr still valid since I was engaged in quran reading? I am now resorting to taking the pill to help me monitor clean and unclean days according to the active pills and placebos.