Sometimes fluid will resorb on its own.
In kids, we can wait up to 3 months to see if it will resorb, providing it doesn't get re-infected. For adults, the sensation of fluid is so annoying that they often don't want to wait more than a few weeks. I usually offer simple drainage first, then a tube if the fluid recurs. I also check the back of the nose to make sure no there is no mass on eust tube.