This article is from the FAQ, by with numerous contributions by others.
o Which form needs to be filled?
I-539 (Extension of stay/change of status)
o What supporting documents needs to be sent along?
Reason for asking for extension, proof of financial support
(i.e. you have enough financial resources to take care of them), a
copy of their return tickets (to show that their stay is temporary).
o What is the fee?
$ 75. If a spouse is also included in the same form then its only
o How much time does it take?
Depends on the region...about 3-5 weeks.
o What dates are relevant, I mean should we go by the
date the Visas are expiring or by the date the I-94 expires?
THE DATE THE I-94 EXPIRES. The date of expiry of visa is IRRELEVANT.
o When should we apply for the extension? I mean how many weeks in
advance to the I-94/Visa expiration should we apply?
Between 45 to 15 days of the date of expiry of the I-94. Ideally,
1 month before the I-94 expires.