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According to the movie Apollo 13 (1995) the radio blackout on reentry was quite a bit longer than expected.

Very dramatic, yes, but:

Did that really happen, and if so, why was it so much longer than expected (or was there typically that much variance on Apollo flights)?

Sort of, but not for the usual reasons. When you are low on batteries, parachutes are more important than radios.

Remember that they were running off of batteries, everything non-essential was powered off, and those system which were needed were strictly powered on only as needed. There were several communication systems, which all would have been powered in a normal re-entry; many were powered off at various times during 13's re-entry.

In particular, the VHF systems were kept off during the blackout, and had to be turned on manually after the parachutes were confirmed deployed. The radio contact was late because the astronauts had to turn the radio power back on.

From 101:53:00 to 102:02:00 and from 123:05:00 to 123:12:00, the communications system was powered up to the extent necessary to transmit high-bit-rate telemetry data using the omnidirectional antennas. The S-band system was turned on for verification prior to undocking and performed nominally. The VHF/AM and VHF recovery systems were turned on at parachute deployment and operated nominally throughout recovery.

Apollo 13 Mission Report, section 5.4

Update: A typical Apollo blackout lasted about 4 minutes. Due to a shallower re-entry path, Apollo 13's blackout was calculated to last about 4.5 minutes. Flight director Gene Kranz's logs show that it took about 6 minutes to re-establish contact with Apollo 13.

Telemetry was usually the first signal received after the blackout. This article from Smithsonian Air and Space magazine confirms that 13's first signal was telemetry:

With no radio signal, there was "no way to tell" how the crew and ship were faring, Kranz says. "There was no telemetry from Odyssey until the end of blackout," he recalls.

Telemetry would confirm that the spacecraft was intact, and biomedical data would confirm that the crew was alive. However, voice communication would confirm that the crew was conscious and that there were no anomalies. The Air and Space article confirms that there was some relief in mission control upon hearing the crew's voices, but not as dramatic as in the film:

Henry Cooper's 1973 book Thirteen: The Flight That Failed describes the tension: "After three minutes of blackout, Kranz put through a call to [lead retro-fire officer Chuck] Deiterich to find out how much longer they had to wait. Deiterich said it should be over in another thirty seconds. At the end of thirty seconds, there was still no word from the astronauts, and Deiterich began to get concerned. Thirty seconds later, the astronauts still hadn't reported in, and Deiterich was alarmed." Even when they finally heard astronaut Jack Swigert's voice over the radio, confirming that the crew had survived, the controllers didn't say a word, just kept silent until the capsule splashed down in the Pacific nine minutes later, according to Cooper's account.