Generally, an oleophobic coat should be able to last a smartphone's typical 2-year life cycle, however, misuse, poor quality, or just bad conditions could cause it to wear out within months.

Why does oleophobic coating wear off?

"iPhone has a fingerprint-resistant oleophobic (oil-repellant) coating. This coating wears over time with normal usage. Cleaning products and abrasive materials will further diminish the coating and may scratch iPhone." It's recommended to seek service for further assistance.

How do you take care of oleophobic coating?

Grab a 70% isopropyl alcohol wipe and clean off the screen. This method is officially recommended by Apple, but will also work on similar Android models. If you use your phone often, consider disinfecting it twice a day.