Years ago, in the 50's and 60's, ten hours was "strongly suggested" in the publications. A chart of the congregation's numbers was prominently displayed usually to the right or left of the platform. That way if you visited a congregation you could see whose was the better congregation.
10 hours is based on going out "preaching" every Saturday morning if you are a good JW.
9:00 meet for service and discuss day's text, form car groups
9:15 leave for territory
9:30 knock on first door
10:30 coffee break (witness to waitress so can keep counting time)
10:45 back in territory
12:00 stop for the day
2 1/2 hours x 4 Saturdays = 10 hours
Thus they figured the average JW could spend at least 10 hours a month easily in the "preaching."
Of course that doesn't count the 4 hours, 4 return visits and 1 bible study parents could count if they had unbaptized children.
6/1/61 WT p. 328
Each congregation has its hour quota and each Witness strives to meet or exceed it. The average minister in a congregation spends some ten hours monthly in such evangelizing. They believe in "buying out the opportune time" for themselves.—Eph. 5:16.
6/15/61 WTp. 373
If you are really pursuing everlasting life, then your record of field ministry should indicate that. Do you spend at least ten hours each month preaching and teaching the Bible truth to others? Can you imagine Paul being contented with less than ten hours a month even when he found it necessary to work full time making tents?
1/1/62 WTp. 20
Do you devote ten hours a month to this work of talking, or just two and a half hours a week, only twenty minutes a day? Think about it! That is not too much time for a Christian to spend, is it, in order to show he is an imitator of Christ?
3/1/62 WT p. 157
Nor may we content ourselves with but one hour a month if we can devote ten hours to prophesying. Ten hours a month is only one hour every three days, and surely we can find that much time for prophesying.
By 1973 this quota disappears in the publications as indicated by this statement
Kingdom Ministry 2/73 p. 1
Some have said: ‘But we have no quotas now.’ Though the hour requirements for the various branches of pioneer service have not changed, it is true that congregation publishers have no set goal of, say, ten hours per month.