Transcribing is, without doubt, the bane of a PhD student's life. It takes me 8 hours to transcribe a 90 minute interview. So, for 30 minutes, I'd say about 2 and half hours for each. I am a slow typer though, so you may be quicker. I found it really helpful to use MyTomatoes when doing it, so I did it in a series of 25 minute blocks with a 5 minute break in between each - rapid progress with frequent breaks.