What are the characteristics of fitrah (the innate natural disposition) for which a specific time limit is mentioned?

The Shaykh : The rest of the question says: And there are some sunnahs (recommended practices), such as washing the armpits; do they fall within the time limit? And what are the time-bound sunnahs?
Washing the armpits has no relation to the time limit. The time limit mentioned is only for cutting the nails, plucking the armpit hair, and shaving the pubic hair; these are the things for which a time limit of forty days was specified.