One entry, one hour, and the best argument I know for owning less and placing it better.
One entry only. The day is short on purpose.
Go in autumn and go in the first hour. The house is small, the rooms are quiet, and the whole effect depends on being able to stand still in front of a shelf without somebody waiting behind you.
Resist the urge to do the colleges on the same morning. Cambridge will offer you six other things and every one of them will dilute this. Come out, walk by the river, and let it settle before you look at anything else.
It is an easy train from London and an entirely reasonable half-day. I have never regretted going up for one house and coming straight home again.