I've heard 300 as well. People have all sorts of study logs and timelines. I think what matters is what works for you.
Personally, I put in over 400 hours. I'm waiting for the results but at least I felt prepared for the PM Geotech and over-prepared for the AM. Let's hope that translates to a PASS. If I fail, I'll have to wait a year because I have no clue what I would do differently.
It takes me some time to get going so as soon as I sent in the application, I sat on the ASCE webinars in September. They were free through work. That got me thinking about what I knew how to do, what I knew but would be slow doing accurately and what I didn't know at all. Then, I started collecting references and loads (LOADS!) of problem sets. I used a few sets as a pre-test to see what I need to focus on and then started going through the references based on that. I may have put a few hours on weekends doing that. Unfortunately, we have the Seismic and Survey exams in California so my time was split. I didn't start really studying until January. I put in about 36 hrs per week after work and on weekends for 11-12 weeks. The majority of the time, I was doing problem sets. If there was one problem I couldn't do or I got wrong, I went back to it later to learn the theory, why I went wrong, and made a note to myself on my cheat sheet to never do that again. I started out with one reference book and added others as I went along if something was missing. My main goal was to be very familiar with one or two references and to be exposed to so many problems that I wouldn't be surprised.
I started 7 months ago but I put in over 400 hours studying and doing problems in the last 3 months. Some topics I didn't bother to study. No way I would have learned them properly in the hope of scoring one or two questions. I'm over 5 years out of grad school with 10 years field experience and limited conventional design experience and a bit specialized. I must say though that the experience made a lot of the questions much easier. This test was skewed towards geotech. No complaint