Of course, there are no "rules" in painting, but since most beginning painters struggle with similar issues, it can be helpful to stick to a few guidelines just to keep from flopping around reinventing the wheel. I recommend beginning painters try it out for a year or two, it certainly won't hurt. This is it:
Spend the first 20% of your total painting time on the initial drawing stage.
So, before you start a painting, decide how many sessions you plan to spend on it. Often as beginners, we have no idea. Two hours? Twenty? 200? Sometimes you are working with a live model so you know how much total time you will have. Other times the painting duration is limited by the subject. Painting flowers from life are nearly always single-session subjects. For other paintings, you can decide if you plan to work one day, three days, a week, a month, or a year.
Of course, you can just start painting with no plan and add layer after layer until you don't hate your painting any more. If this is working for you and you love your paintings, keep doing that.
But if you don't love your paintings, try this: Estimate how much time you plan to spend, and then spend 20% of that time on the drawing stage.
So, for a 5-day painting, spend one full day on a line drawing.
For a 5-week painting, spend an entire week on the preliminary drawing (I do a detailed graphite line drawing).
For a 5-hour alla prima (single-session) commit to spending the first whole hour using only umber paint, making an accurate under-painting.
For a painting duration not measured in fives, you’ll need to do a little math to calculate 20%. For a single 3-hour portrait session with a live model, I spend the first two 20-minute sessions, a total of 40 minutes, using only one color of paint to map out the shapes and values.
Why 20%?
A painting relies on the structure of the initial drawing. A one-hour drawing will not be accurate enough to support 49 more hours of painting, but a one-hour drawing is plenty of structural support for four more hours of fresh, lively paint application with large-ish brushes.
If you work much longer, you will be required to add a level of detail and refinement that the drawing is not able to support.
The inaccuracies and generalities of a 1-hour drawing will be glaringly revealed.
When you see your drawing errors, you will try to "fix" your painting. As soon as you start "fixing" mode, a painting starts a death spiral.
At this point, it's better to put the painting aside, take what you learned, and apply those lessons to the next painting.
Notice in the first 1/5th of this time-lapse video I am only drawing with umber paint, before I start adding the lead white. I consider the umber stage the "drawing" stage.
All of my online painting courses also share lots of information about my method and approach to drawing.