Mission Control was out of contact with the Apollo capsules orbiting the moon for about 45 minutes out of every 2 hour rev. That figure is from memory, so it could be off but it's in the ballpark.

Of course, everyone knew about the communications outages ahead of time but there was still reason for concern. A lot could go wrong in 45 minutes.

I do remember reading how the mission planners calculated certain critical return of signal times as indicators to success. For example, had the Service Module engine not fired to insert into lunar orbit, they would've regained contact several minutes earlier than if it was in orbit. That served as an early success indicator. Likewise, had the Service Module not fired to break lunar orbit for the return trip, they would've regained signal several minutes later than expected.

In the book that inspired the excellent "From the Earth to the Moon" series, there was a lot of discussion on the Apollo 8 chapter about Mrs. Borman's fears regarding the Service Module engine. She was convinced that the engine wouldn't fire for the return trip, leaving the men stranded to die in lunar orbit. She was battling alcoholism at the time, perhaps due to the stresses of her husband's occupation.

Fortunately, all of the Service Module thruster firings happened without incident. It was a very simple propulsion system designed for maximum reliability instead of maximum possible performance, so there wasn't much to go wrong. The SM used bipropellant hypergolics in pressurized tanks, so all that had to happen was for the fuel valves to open (same as on the Lunar Module Ascent and Descent stages). There were no complicated turbopumps or igniters to fail.