300 hours a year? Even small schools it’s pretty easy to get double that. Triple that at a busy school.Provided, you are going to be working your non-aviation job while you do flight training concurrently, Its going to take a little while to get everything. At age 51, lets say it takes 2 years to get all your ratings, you're still going to need to acquire 1500 of TT to be eligible to be hired by a regional carrier. Getting 1500hrs is no easy task, especially if its CFIing at a part 61 school, itll take years. Id estimate 300 hours a year would be good (provided you quit your current job and go all in). Now we're into 55-58 years old before you're qualified for a regional airline job. By the time you make left seat and starting finally getting up to a more respectable quality of life and paycheck, you're in your 60s, and by then, (even if you got hired by a major) would you even want to start over again knowing you may retire in the right seat with an undesirable schedule. Sure all of this is "possible" but id say its unlikely and probably unwise.