Does 20 hours seem like a long time to work on a 20 minute piece that needs dialogue mixing, music mixed in, and sound design including a few fight scenes?
Crazy fast. To do the dialog editing well, it could take an entire day. That depends largely on how well it was recorded in the first place and how many disasters were created in the video edit room. I have clients that know a 20-minute episode is going to take a solid week of laborious sound effects editing. If the music is prepared well and cleanly laid out, that can be done in a couple of hours. But if there are songs, or pieces of source music, or poor edits, that could take all day.
Even if you're not a member (and I'm not), read this to get a sense of wages. Keep in mind that you have to pay your own benefits as an independent contractor. AND, keep in mind that you will quickly set a precedent with a new client when it comes to pricing. Do it once too cheaply and you will be a slave... https://www.editorsguild.com/index.cfm