Yes the expected latency for ISBD link was about 1-10 minutes a while ago. See my measurements of ISBD signal quality at https://discuss.ardupilot.org/t/global-satellite-telemetry-for-ardupilot/18452/22 for more details.
But a few months ago Iridium replaced the 20 years old satellites by Iridium NEXT constellation. So the things might have changed for better. The old constellation had a few non-operational satellites, that might have contributed to the gaps in signal quality. And, I hope, the satellite technology made some progress during the last 20 years. I did not see long gaps in my recent tests, but I did not test it extensively.
Also, if you target less than 1 minute latency, you need to power RockBLOCK directly using 3.7v pin. Send-receive sessions drain the supercapacitor, so RockBLOCK can do only about 1 session per minute, if you power it using 5v pin with limited current. I did not try powering RockBLOCK using 3.7v yet.