A better question is how accurate does it need to be? If it’s a portrait and you want a likeness, it needs to be pretty accurate. Humans are very good at recognising faces. A slight difference in the angle of the eyes, or the distance between the eyes can make or break a likeness. This just takes practice. Since we are so good at recognising faces we are equally good at seeing the differences when we draw them, so practice.
I am reasonably good at getting a likeness but I struggle if it’s someone I know well. I usually use a photograph and replicate the relative dimensions as accurately as I can. I am very jealous of artists who can draw a face with a few deft strokes and get a recognisable likeness. With plants and landscapes accuracy isn’t really an issue. If it really doesn’t look right I paint over it with as many iterations as it takes to fix it. With animals accuracy can be an issue but it just has to look like a horse, if it’s someone’s pet that gets a little different. Years ago I did a pen and ink drawing of my wife’s dog. I didn’t like it at all but I’d worked on it long enough that rather than throw it out I just finished it quickly. I went back to it later expecting to toss it out and start over but it was great. One of the best I have ever done and my wife loved it. I’ve attached a photo of the drawing to this post. It looks almost unfinished however it looks exactly like my wife’s dog. I worked for a long time on the eyes and nose. The fur and ears were added very quickly.
The moral of this story is; if you have been working on a portrait for hours, don’t assume that you can see what’s wrong with it. Put it aside or get someone else to look at it. As artists we become visually fatigued and we need a fresh viewpoint. Many artists set the work aside and go back to it a few days later. I’m too impatient for that so I use my wife and daughters to critique it. This is hard (my family is brutal) but it works.