Started to learn this serve during 1st COVID lockdown, March/April 2020, by August 2020 I had a 'reasonable' reverse pendulum serve.
So it took me about 5-6 months or so, and at that time I was practicing serves at least every other day.
Everyone is different, for some it could take way less time!!! Adult learners may not be a quick to pick things up compared to juniors.
By reasonable, I mean that I could vary spin, back-side/side/top-side with a half long serve down the line, and had a good fast serve to far corner, usually with top/side spin, or side spin. long fast back-side spin serve was ok, but harder to get the same speed compared to top-side/side spin.
I have found that you have to practice ALL of your serves generally, but the reverse pendulum needs a little more attention!!! 3yrs on and I haven't made much improvement to my FH reverse pendulum serve, I've concentrated more on my hook serve.
To really get to grips with the reverse pendulum serve expect to put the hours in to start with!!!