The default is 30 minutes, the timer can be increased system wide to 120 minutes in Voice Processor, Timers and Limits, Timers and Limits, by setting the Record-A-Call Max Message Length timer.
Note that if you are using voicemail to email, recording conversations over 20 minutes in length can exponentially increase the processing time. I have a relatively small customer (30 phones) that record 60 minutes calls frequently, there is often a 20-45 minutes turn around time before the message is sent.
