15 minute rule is for older or ill jws that can't put more time in. They have to be approved by the congregation service committee.
It is not for younger, healthier jws. But local jws make their own rules at times.
Km 3/02 p.5 par.6 "Gifts in Men" Eagerly
Shepherd God’s Flock
6 Valuing the Infirm:
Loving undershepherds are also concerned about those who could be overlooked.
Associated with every congregation are some who are infirm, confined to nursing
homes, or otherwise incapacitated. Their share in proclaiming the Kingdom
message is understandably limited because of their circumstances. Perhaps their
opportunity to speak out may come only through contacts with visitors, other
patients, or caregivers. Nevertheless, whatever they are able to do is viewed
as a valuable contribution to the overall preaching work. (Matt. 25:15) Even if
they witness for just 15 minutes, this should be reported, and they will
continue to be counted as regular Kingdom publishers.
Km 10/0 2p.8 par.6 How Congregation Book Study
Overseers Show Personal Interest
6 As a Loving Shepherd:
The book study overseer is interested in those who because of their
circumstances are able to have only a small share in the preaching work. He
makes sure that those who are very limited because of advanced age or because
of being shut-ins and those who are temporarily limited because of serious
illness or injury are aware of the provision that allows them to report field
service time in 15-minute increments if they are not able to report a complete
hour during one month. (The Congregation Service Committee determines who
qualifies for this provision.) He also takes an interest in those assigned to
the group who may be inactive, striving to help them resume their activity with
the congregation.—Luke 15:4-7.
I do believe a letter was
sent to all elders in 2001.