The conversion takes place in the "cloud" and it takes between a few seconds and a minute. The delay comes when Moodle goes to pick up the recorded file. Rather than have the user wait for their recording to finish converting, we schedule Moodle to go and pick it up when the "cron" job runs. Cron is a background task on the server that wakes up every x minutes and does what is scheduled, and then goes away again.
So I think you have one of two situations:i) your cron job is not running, or is running more than every 30 minutes

ii) you are not using a Poodll player to play your audio.
This is a problem, because Moodle will cache the placeholder audio "Poodll is converting your..." and play it back even when the converted audio has been picked up.