Dhikr is to be Performed as Much as Possible – Ibn Abbas in Tafseer Ibn Kathir
Alee ibn Abee Talhah (rahimahullah) reported that Ibn Abbas (Allah be pleased with them both) stated regarding Allah The Most High’s statement
اذكروا الله ذكرا كثيرا
Remember Allah with much remembrance. (Al-Ahzaab:41)
Verily, Allah did not obligate an obligation upon his servants except that he legislated a fixed limit for it (1), and then excused its performers when they are in a situation when they are excused (2), except in the case of dhikr (remembrance of Allah) for indeed Allah did not legislate a limit for it and nor is a person excused for abandoning it, except when compelled. Allah said,
فَٱذۡڪُرُواْ ٱللَّهَ قِيَـٰمً۬ا وَقُعُودً۬ا وَعَلَىٰ جُنُوبِڪُمۡ
remember Allah standing, sitting down, and (lying down) on your sides (An-Nisaa:103)
night and day, at land or sea, whilst travelling or resident, when wealthy and in times of poverty, in sickness and in health, in private and public and in all circumstances, He, The Mighty and Majestic said,
و سبحوه بكرة و أصيلا
And glorify His Praises morning and afternoon [the early morning (Fajr) and ‘Asr prayers]. (Al-Ahzaab:42)
so if you were to do that (in the manner mentioned by ibn Abbas (Allah be pleased with them both)) then Allah will send His Blessings upon you, and the Angels will supplicate for you.
Tafsir Al-Qur’aan Al-‘Adheem ibn Katheer vol.11 pgs.182-183
(1) e.g. obligatory prayers are a fixed number (five) with set number of raka’aat, obligatory fasts of Ramadhan are one month every calendar year.
(2) e.g. a traveller, sick person does not have to fast obligatory fasts, the woman on her menses does not pray or fast etc