I emailed 1Kvoice about mechanical problems that I encountered on a number of flights last fall and this is part of the reply I received. Hope that answers some of your questions.

"I hope that the following information helps to address your concerns regarding our maintenance schedule. In compliance with FAA Regulations, United’s aircraft receive an “A” check every 300 flight hours, which translates to about every 16 days. An “A” check is an extensive review of all the major systems on an aircraft. In addition to this, overnight and other routine service checks are made weekly. We also perform “C” checks in which every component of the aircraft is thoroughly inspected by highly qualified system-specific inspectors. United ensures that all of our aircraft are in the safest operating condition possible. Rest assured that safety will always be our number one focus."