Fifteen Minutes a Day to Read
All you need is 15 minutes a day to read a book. I'm speaking from experience. I've finished two books reading just 15 minutes a day, some days even less. Sure, you could spend more time reading. That will allow you to finish more books in a shorter amount of time. But, if you're already struggling to keep the habit of reading, telling yourself you should spend even more time reading won't help at all.
One thing I've learned from the Atomic Habits book, is that we're more likely to keep up with a habit if it is easy and convenient. The less effort it takes, the more likely we can keep up with the habit. If 15 minutes is the most you can spend reading, then by all means take it. You either spend 15 minutes or less reading, or you'll end up not reading at all.