Maryna,
Started studying for REG and FAR in mid-May 2017, tested REG in mid-July 2017 and FAR in mid-August 2017. I accomplished this using Roger. I would stream his lessons, skipping all the question review lectures, on my commute into and away from work, ~1.5 hours each day. I would also drill MCQ and TBS questions for ~1 each morning. My days ran from ~4am to ~8pm, arriving at work around 5am, and studying/breakfast until close to 7am, then working from 7-4 or 5, and driving home, dinner and bed. It was difficult to focus effectively every single day, so there was a lot of time in that space that wasn't as productive as it could have been
I also scheduled 3 hours of studying each Saturday and Sunday but wasn't able to hit that consistently. It was hellish and after I finished I hit a three month long serious depression. I would not recommend it, but it is, in theory, doable. I think I had modeled out that it was roughly 250 hours of prep, and I was averaging between 15 and 20 hours of study time per week. In theory I could have hit 25 hours on that schedule.
My point is, you can do it, but it will be very challenging, and if you only have one passing score in the bag and it's about to expire, then you may be better served assuming you'll lose the credit and developing a longer-term strategy for passing all four.
The way I approached mine was to recognize that there are only 6 quarters (opportunities) to take each test in a rolling 18 month window. It should be a goal to take at least one test in each subsequent testing window, allowing you ~2.5 months to prepare for each one. On the last two, I accelerated because I anticipated a difficult Q4 and did not want to be running up against an expiration deadline in Q1/Q2 of 2018. Just a few thoughts. Good luck however you go.