There is no ratio. It completely depends on your hardware, the media, the effects you have on your timeline, and whether or not you have GPU-acceleration enabled.
Hardware issues could be:
- Slow processor
- Not enough RAM
- Storage drive is too slow or too full (usually drive speed is the bigger issue)
- Old or unsupported GPU (Premiere requires the GPU to have a minimum of 1 GB of VRAM in order to use GPU-acceleration)
Media issues could be:
- Highly compressed video which is processor-intensive to decode, such as H.264. This can be rectified by converting to an easier-to-edit intermediate format, such as ProRes, DNxHD, or Cineform before importing, or by using the Proxy workflow Work offline using proxy media |
- You didn't say what format you were exporting to, which is another factor. If you're going from one highly compressed format to another (h.264 back to h.264) then export speeds will be much slower than if you went from h.264 to and intermediate or vice-versa
Effects issues could be:
- You have a lot of effects on your clips
- You do have GPU-acceleration enabled, but you're not using GPU-accelerated effects, so the CPU has to process all of those
- You do have GPU-acceleration enabled AND you're using GPU-accelerated effects, but your GPU is still old or you're overtaxing it (there's a lot of math going on here and if you tax your GPU then it will be sad)
In short, and as usual, it depends. You have to give a whole lot more information for a more tailored response.
Complete hardware and software (both OS and version of Premiere), as well as what media you're using and what you're trying to export to.