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. The LED current required depends very much on the load on the opto-transistor collector. Perhaps those problems were a result of poor design in the past. With a 10k collector load the 5mA LED current should be nearly double what is needed (according to the datasheet) to drive the transistor into saturation and trigger the 555 if using a 12V supply.
As you already have the 4N35, convince yourself this works by trying a simple experiment. Pass 5mA through its LED and measure the collector voltage when using a 10k collector resistor fed from a 6V or greater supply. The answer should be << 1V. Anything less than 1/3 of the intended timing circuit supply voltage would be enough to trigger the 555.
Edit: The only other reason I can think of for using a higher LED current would be to reduce sensitivity to noise pick-up. My suggested circuit already has noise filtering, but you could beef that up by connecting a capacitor (say 100nF) directly across the LED.