Question: How many days should a Pure Land wake be?
Answer: There is no fixed duration.
Q: Why are many wakes usually of three or five days?
A: For Chinese, this is due to following the custom of having wakes ranging from 3 to 7 days, for an odd number of days, as even numbers are reserved for joyous occasions, symbolic of togetherness instead of individual departures. 3 or 5 days are usually chosen as 7 days or beyond needs considerable more time, energy and expenses. As this is a non-Buddhist custom, it need not be stuck to rigidly.
Q: Is there a minimal number of days?
A: The wake should offer enough time for the consciousness of the deceased, who might still be around, to have enough guidance for Nianfo (mindfulness of the name of Amituofo) practice to reach his Pure Land. Depending on nature of the passing, the number of days should be neither too few nor many. For instance, one who died unpeacefully should be offered more time (such as 5 days), though one who is likely to become more attached to wanting to stay indefinitely (though karmically impossible in the long run) with the passing of time be offered less time (such as 3 days). Near the body of the deceased, the number of days left before cremation or burial should be periodically reminded, so as to urge more sincere and diligent Nianfo.
Q: Must a casket be expensive for the good of the survivors (i.e. the next generation)?
A: If the lack of a proper casket is due to disrespect and/or miserliness, such that it disturbs the consciousness of the deceased, who might still be around, this means the survivors create negative karma for themselves to some extent. However, the coffin need not be lavish; just good enough to not cause possible displeasure.