How much input current should be passed for switch operation?

The current with the threshold value for input current IF that reverses the state of the output MOSFET to perform output-side operation is defined as "Trigger LED Current IFT". For usage, the driving LED current must be set to a value equal to or greater than "IFT(max)" written in the technical documents. In Toshiba's lineup of products, IFT(max) is 5mA or less. In addition, this IFT may be affected by deterioration over time and other change factors within the ambient temperature range. Consideration must be also given to the coefficients of variation for these factors.
For details, refer to Toshiba Application Note "Photorelay Specifications" Chapter 5. DESIGN CONSIDERATIONS.