Question: Should post-death practices to benefit the deceased be within 49 or 100 days?
Answer: In the Buddhist teachings, rebirth is taught to usually occur within 49 days after death (if in the bardo state), while there is no mention of anything significant about the duration of 100 days, with belief in the latter probably arising from Chinese, but not Buddhist culture.
It is important to have greater focus on the 49 days as there will be a more appropriate sense of urgency to aid the deceased with offering of guidance, support-chanting and merits. If imagining there is the ‘wider’ window period of 100 days, there might be ‘dangerous’ procrastination instead.
Of course, offering the above prayers and such beyond 49 days is harmless, but this will be less directly beneficial if the deceased is already reborn. Guidance and support-chanting might not be heard if already reborn elsewhere, although merits dedicated can still alleviate possible suffering (by diluting negative karma while increasing blessings) in the new life, and expedite a better rebirth, if born in a lower realm (as a hell-being, hungry ghost or animal).