It is just a quote from Pascal's Thoughts so more like haiku or ad hoc observation then a commandment.
You don't need to sit 24 hours alone in an empty room - you just should be able to - this is my interpretation.
It is like '- Master, how long should I meditate? - 20 minutes is enough. - But I don't have time for that. - If you don't have time then meditate for one hour.'