The better the times, the harder to PR.
It also depends on the overall skill. If you only played LC for months and got regionals, you wont't get a good time on DDR.
It is all about knowing the strats/tech. If you fail your runs on the first turn, you have to ask yourself:
1. Why am I so bad?

1. Why do I fail there? &
2. How to improve it.
I used to play Custom Tracks. They are short, uncomplitcated and you learn the technical aspects of the game really fast. You also have to watch/read about strats and WRs and try to understand why pro players do it.
To answer the question, I PR everyday (in any form, flaps, best splits, 3lap) and my sessions are 1h - 3h long per day.
In the past I used to player 10h a day but did not PR.
Endless sessions kill your progressI choose random categories and when I notice that I don't hit my average laps, I pick another track