I took, and passed, the PE exam in April (Power, not water resource though). I started studying Jan 1st, so 3.5 months before the exam. Here was my schedule:
3 days a week I stayed late at my office (quiet and alone) after work was done for the day and stayed 3-4 hours each of those nights, reading/studying one night and doing a 1/2 practice exam (4 hour 40 question) for the other 2 nights. And every Saturday I spent 8 hours at the library doing a full practice exam (8 hr 80 question). I did that for 3 months, with occasional schedule changes where needed due to work/life, but I made up any missed day on a different day. So, ~20hs/week for 3 months. I was committed!
The week of the exam I worked Monday, but I took Tues-Fri off to do finally prep and study. I retook the NCEES official practice test Tues, another full test on Weds, Thurs I took a 1/2 exam in the morning and drove to my hotel by the test site, then reviewed my notes, had a good dinner and went to bed. Got up bright and early on Fri, arrived at test center early and nailed it. Then I went home and had a nice dinner with my family and cracked a celebratory beer. All total I logged 250-300 hrs. Maybe it was over kill, but I wanted to be confident I'd only have to go through this once and only once. I feel for those on their 2nd, 3rd, 4th attempts. I figure if I couldn't commit to find the time to study right the first time, how would I be able to find time for another 3 months, or another after that. I figure go hard, and be one-and-done
ymmv
Good luck! You can do it!