How much input current should be set for turning the triac ON/OFF?

The current with the threshold value for the photo triac to turn ON when the current passed to the LED is increased is defined as trigger LED current (I_FT). It varies with the product. The maximum value for such a variation is specified with I_FT(Max). However, this does not mean that in circuit designing, LED current I_F must not exceed this Max value. In designing I_F, , also by considering LED deterioration over time, temperature, and design margin, a value for I_F larger than I_FT (Max) must be set.
For details, also refer to Toshiba Application Note "Triac couplers-basic properties and application design", section 9.1 How to design the input-side LED current I_F. For I_F, the maximum rating including temperature derating is also specified, and whether or not such a rating is exceeded must be checked.
(https://toshiba.semicon-storage.com/ap-en/semiconductor/knowledge/application-note.html)