Divide your age by two and add a few hours for good measure.
This is a very rough and ready estimate though - it depends on your individual aptitude (which can widely vary at different stages in training), the amount of continuity you acheive (often depending on weather and other factors beyond your control), whether or you're training at a busy international airport with three frequencies and frequent holds before getting airborne or a quiet GA field in the sticks etc, etc ...
Some people take a bit longer to solo and then rattle through the rest of the course and get their PPL at first attempt in minimum hours, others go solo in a prodigously short time and yet require 60 hours or more to qualify.
The important thing is not to get too fixated about this.