For new brewers, longer is usually better, as new brewers usually aren't pitching enough, aerating properly, or properly controlling ferment properly. However, when fermentation management is on point, no reason beer can't be done very quickly. But it's also very important for new brewers to actually check to make sure gravity is stable.
I usually package at 2 or 3 weeks, depending on the beer, but that's more because of when I have time to bottle. If I could keg, I'd probably do some sooner. I have some beers (my house Mild and Bitter) that are usually ready for packaging by about 5 days after pitching yeast. I just don't usually package then, but no reason I couldn't.
The key lesson is that the beer is done when it's done, and done doesn't just mean "at final gravity". There's some cleanup too. However, at lot of folks parrot the idea that beer needs longer to clean up than it actually does. In a proper fermentation, the cleanup phase should be finished 24-48 hours after reaching FG. If the fermentation management isn't good (poor pitch rate, poor yeast health, poor aeration, poor temp control, or any combo thereof) then it can take longer to reach FG and take longer to condition afterwards.
Point is, if it's not done, you need to wait until it is. Stable gravity, plus no residual compounds like acetaldehyde and diacetyl (or in some cases where they're allowed acceptable levels of). If the gravity is stable and the beer doesn't taste green, then you can go ahead and package it. No point waiting longer.
Eventually you'll reach a point where you know about where the gravity should finish (especially when brewing something you've brewed before), so you know right away if it's not done or if there's a problem, and if it's about where it should be, you can tell by taste if it's done or still working. Until you reach that point, I'd suggest confirming 3 days of unchanged gravity before packaging.
