There really is no set answer on this but I would wait until you know you can perform your normal routine without any problems. If you think your going to struggle because you still have symptoms, your better off staying away from the gym.

If you try to come back too soon you could prolong your sickness because your putting unneeded stress on your body whose immune system is already working over time. So worry about recovering first, then worry about getting back into your routine. A week or two away is not going to kill you.

Not to mention if your still coughing etc you risk spreading it to everyone else at the gym.