In principle, if it doesn't matter, if the idea is just that this happened sometime in the past, then you don't need to bog the story down with details about exactly when. I certainly wouldn't go into some long description if it doesn't matter. I mean, I wouldn't say, "In the third year after she graduated college, on the tenth of June, at 3:15 in the afternoon, ..." if none of that makes any difference.
That said, I'd add two big caveats:
(1) You need to make clear that this is a flashback. I've read a few stories that had a flashback with no introduction, I didn't see any clue that this was a flashback and not ongoing narrative, and I had to read quite a way before I realized, Oh, wait, this is a flashback. In one case I was about 3/4 of the way through a book before I realized that the entire story was a flashback from the first chapter. I was puzzled about how the events in that first chapter fit with everything that happened -- I thought -- after it. I think that kind of ambiguity is very, very bad. It breaks the reader's connection to the story. And usually the easiest way to tell the reader that something is a flashback to throw in a time reference. Be very careful about saying, for example, Well, it's obvious that it's a flashback because I say, "As she entered the high school ..." As a reader, just telling me that she entered a high school doesn't tell me that this is a flashback to when she was a teenager. She might be going to a high school re-union, visiting her children's school, attending some community event at the school, etc etc. Especially if you say that anything after the first sentence of the scene makes clear it's a flashback. If I have to read three paragraphs before it's obvious, that means that I then have to go back and revisualize those three paragraphs.
(2) While it would vary with the story, I think you almost always need to establish at least a general time frame. Is this happening when she is 5 years old or when she is 25? Is flashback A before flashback B or after? Etc. As you are writing you no doubt have a picture of the scene in your mind. But the reader can't see that scene unless you tell him. Again, you want to avoid having the reader have a certain mental picture of the scene, and then say something that invalidates that scene. Like, if you just tell us "Many years ago, she ...", I might in my mind picture a woman of 25 going through whatever events. Then if suddenly you tell me that her mother told her to put away the coloring book and go to bed because it's a school night, I'm going to get ripped out of the story and have to go back and re-visualize the scene I've just been through.
There are times when you deliberately want to confuse the timeline. Like if the idea of the story is that the character is an old man who is lost in his memories and can't distinguish the present from the past. But any ambiguity in a story should be careful and deliberate, not an accident.