Yes, I'm talking about one of these simple, robust and boring strategies. This has nothing to do with the more finer nuances of poker (being able to identify and succesfully target weaker players) or even most of the very basics. It's strictly automatic play, where you have a certain set of rules that you obey each and every time for the optimal outcome in the long run.
This includes only choosing a suitable table to join, the size of your buy-in, when to reload, when to leave the table, recognizing what your position is at the table, what starting hands to play, and how to play them every single time, and when to move up (or down) levels. It is not required to understand why you do what you do, all one needs to do is to follow the instructions blindly. You can keep one A4 paper at your side at all times from where you can check everytime you can't remember what you are supposed to do in any given situation.
For a beginner*, 3 days (or 9 hours of play time) is probably enough for them to learn these simple steps by heart. In my opinion / experience a beginner can start playing after 30 minutes with the paper next to them without making too many mistakes, that is the maximum time I estimate it takes a normal person to understand and apply the instructions. A more experienced recreational player can probably grasp the strategy in 10 minutes.
* Someone who already knows how to use the online poker software and is familiar with a deck of cards.
That sounds awfully low, what is your criteria for a professional? For me it is someone who can earn their living (rent + food + little to spare) playing only poker.
Btw. I'm strictly a recreational player (balance heavily on fun rather than income), but would like to step up my cash game. So far (3-4 years) I've been playing only by feel. What are the first important things you think I should look into?