there is this professional brewer on youtube and he says that you really do not need to ferment for 3 weeks despite everyone telling you you should. Apparently because that is an old saying and it is found to not be necessary anymore. I have made good beer before fermenting for only 7 days too. I just want to be extra careful this time because my last batch messed up somehow
Keep in mind, the professionals are typically kegging it, whereas some homebrewers don't keg but are bottling it. If you keg it, you can likely get away with kegging it early. But, if you are bottling it, and you move it into bottles too soon... you've got potential gushers or bombs on your hands.
Regardless, there is NO SET NUMBER OF DAYS FOR ANY FERMENTATION. Yeast is a living organism, and it is affected by hundreds of variables. Yeast will work on its own schedule, not always on the schedule that humans desire. Don't ever forget it.
Many Belgian yeasts are extremely fussy and can take 3-5 weeks to complete fermentation. German ale yeasts sometimes take 5-7 weeks. On the other hand, some British yeasts are consistently finished in exactly 36-40 hours every single time.
There is no consistent rule of thumb for most yeasts. Let the specific gravity guide you. The best advice I can give on this, which I've said in the past and I'll say it again:
Wait until you think fermentation is definitely complete. Then check gravity. Then wait at least 3-4 more days. Then check again. If you were right and the gravity did not change by even one single point, then it's done and safe to package. If not, then fermentation is NOT complete, and you need to continue the cycle every 3-4 days until it's stable.
Unless you're kegging. This guidance is true but is more important when bottling. If you're kegging, you can get away with sins to some extent, at the expense perhaps of a few pints of hazy yeasty beer for a few days, and potential for "green" flavors including extra sulfur or diacetyl.