It depends, but here's how you can tell...
It's good practice to hone knives often (I recommend once a day or before you use the knife for a session).
Honing helps center the edge of the knife, prevent edge folding and nicking, and provides structural support for the edge.
Despite regular honing, a knife blade will eventually wear through a variety of dynamics: folding, nicking, crystal deterioration, and oxidation. At this point, the knife will need to be sharpened.
An easy way to tell if your knife needs sharpening is:
If, despite regular honing, your knife still doesn't feel sharp, then the blade is worn and should be sent for professional sharpening
Unless you are very skilled, I don't recommend sharpening at home. You are unlikely to be able to remove the right amount of material and restore a consistent edge bevel to the blade....as a result the knife edge will not be properly aligned and the knife will go dull quickly again.
The period between sharpening varies a lot depending on what kind of steel your knife uses, its blade geometry, your cutting technique, the food products cut, how often you use the blade, and of course how often it's honed.