Maths could probably be quicker (in school we did it in about 6 or 7 months), but biology and chemistry have so much content to memorise that the standard 2 years is more likely no matter how quickly you understand it.
How about 2300 hrs so approx 575 days or 1.5 years?
if you are starting from September next year with a view to sit your exams in 2023, then that's the usual timeframe and you don't have anything to worry about.
If you want to start now with a view to sit your exams in 2022, that'll be a lot more time-pressured but could still be possible. I'd ask someone who knows your ability better.
Now I am curious, has anyone studies three a levels from scratch in a year and a half time? So if you learn the content/ study for 4 hrs per day?
Considering most people do them over two years in school with 3-5 hours of taught lessons a day, and then several hours of extra study every night/at the weekend, you'd probably be cutting it fine only having 4 hours a day to put in.