Opto-characteristic for schematic
You need to look at this table in the data sheet: -



Look at the bottom row - it's telling you that for a forward current of 8.4mA, providing you don't take more than 2.4mA through the collector, you can expect a volt drop from collector to emitter of 0.4V max.

How does this work with your circuit? 2.4mA through R10 will generate a gate voltage of 24 volts but of course this is not possible with a 12V supply - this means the opto's transistor will be even more saturated - that is good normally but, you have another 10k resistor (R6) in series with the collector - this will limit the drive voltage on the gate to about 6V. I'm mentioning this because I don't recognize the MOSFET number and cannot tell you if this MOSFET is suitable with this gate voltage.

If  you used a forward current of 1mA and a collector current of 0.2mA the saturation voltage is typically 0.2V but, 0.2mA through R10 will only produce 2V at the gate and this certainly may not be enough to adequately turn it on.

So, given that all we have is the data sheet you are probably looking to drive at least 2mA thru the diode BUT the outcome is pretty much determined by the MOSFET. Also, why are you using R6 - it doesn't seem to serve a purpose.