Low resolution previews are quick, usually it's around a minute of rendering time per minute of audio
(E.g. 4 minute audio track = 4 min render, approx)
Purchased videos are different, they have a much higher resolution and a bigger output file size (Some styles can produce videos that are 4GB+)
The overall rendering time will vary quite a bit, all depending on the type of style you have chosen (some styles are more complex than others), the amount of clips you have added and finally how many overlays you have added.
E.g. A 5 minute video, using 80 clips and 60 text overlays could take 30 mins+