If you have the common flu then there's nothing a doctor can do for the flu other than giving you some medicine to help fight off the flu virus.
They will also just recommend getting plenty of rest, Vitamin C, stay hydrated etc.
If you go to the doctor before the flu virus infects you too much then you can get prescribed some Tamiflu but you must take the Tamiflu 48 hours before the onset of the flu.
After the flu sets in there's not too much you can do other than stay hydrated, rest and get plenty of vitamin c etc.
It takes around a week to 14 days to get over the flu but if the flu lasts longer than 14 days or you think the flu is getting worse then it would be a good idea to see the doctor or go to the emergency room or urgent care.
But if it's just the regular flu then it's best to wait it out for at least a week or 10 days and after that a doctors visit would be a good idea.
But if you can get Tamiflu 48 hours before the flu sets in then you may not even be infected with the flu and it will go away sooner.