Om my MacBook Air I started a flight recording of one minute (didn't change the default value). I added a lot of events, but everything seemed fine.
Later, I started another recording, now with even more events enabled. I didn't touch the duration this time either, only what events were included.
The recording started, the dialog showed a familiar progress bar for a recording of length 1 minute – but in the JVM Browser, under the Flight Recording node, it said that the recording I just started had about 19 minutes left to be completed.

JDK: 9, build 85
JMC: 6.0.0-402
OSX: 10.10.4