There is no formula for that. Read as you need to. Have your mentor(s) suggest books that are specific to your needs. A good starting point might be one relevant book per month to start. There are also those book summary services that can give you the themes of the books in 5 minutes of reading. Subscribe to that and then you can get the full versions of the books you connect with best.
The right question here really is to ask what you should be reading. Once you figure that out, you can plan how much time you want to spend on it given all of your other responsibilities as a startup CEO.
These days, it is very easy to spend an entire day reading interesting things without getting anything useful done! :-)
P.S. - I myself am a voracious reader, but try hard to practice what I am preaching above.