It is very difficult to predict the reasonable period between turnarounds for an expander because experience has varied greatly across the industry. Some practitioners can successfully make a 5 year turnaround while many struggle to keep the expander online for more than a year. Usually, the culprit causing is deposition of catalyst fines on the expander blades which in turn causes vibration. In the past decade much improvment in blade coatings and machine design have been achieved. Your expander manufacturer can likely discuss these options with you. From a process perspective, the key to achieving long run lengths is to always operate the expander at its design point. This is because a properly designed expander will experience uniform flow throughout the flow path which will help prevent deposits. Operating above or below the design point may allow eddy currents and dead spaces to develop which will foster blade erosion or allow deposits to form. This means that the unit should be designed such that some flow to the bypass is required even when the unit is at design feed rate so that control of the flow through the expander can be maintained.
Routine cleaning of the expander (with rice, walnut shells, etc) is also important. Don’t wait for deposits to become a problem because these can also affect the flow pattern and prevent the cleaning agent from getting where it is needed. Many practitioners clean on a weekly basis.