What is the ruling on setting a specific time limit (such as 21 days) for performing acts of worship like Dhikr and reading the Quran, with the aim of challenging oneself and getting accustomed to them? And is this considered an innovation (bid'ah)?

Obligating oneself to specific regular devotions (awrad), such as completing the recitation of the Qur’an (khatm al-Qur'an) and increasing remembrance of Allah (dhikr), is among the good deeds. The more a servant engages in these, the more complete their reward will be. So, if a servant obligates themselves, as a form of training for their soul in worship, to complete the Qur’an within a specific period—such as twenty-one days, or less, or more—whereby they have a defined daily devotion that they do not neglect, then this is very good and is not described as an innovation (bid'ah).