There is no optimal period to add the the light. The birds don't care. The simplest is to add the light in the morning so they can use a natural sunset to go to roost at night. This means to maintain the same period of light you have to adjust your time clock every week or two to keep up with the seasonal change.
Or you can choose a set period every day. The period that makes the most sense is the time from sunrise to sunset on the longest day of the year at your location. This means as the days get shorter you are supplementing light in the morning and evening and as the days get longer you end up back where you started with a full day of natural sunlight. If you don't wish to leave the lights on for the entire period you can use a photocell in conjunction with a timer. The timer will switch the lights on in the morning, the photocell will shut them off at dawn and turn them on at dusk, and finally the the timer will switch them off late in the evening.
Using supplemental light in the evening is going to require a night light that is either left on all the time, a night light that is switched off 20 minutes or so after the lights go out, or a dimmer that can simulate sunset. Plunging them into darkness is not a good idea, they may pile up or at least end up sleeping on the floor.
On a side note, in a completely dark layer barn it is possible to run the lights in "blocks" of time. Light 2.5 hrs on and then 1.5 hrs off, or so, six times a day. As long as the the total amount of light in the day is 14 to 16 hours they continue to lay as normal. It has the effect of desynchronizing oviposition times so that the flock lays eggs spread out throughout the 24 hour period, instead of laying 80% of the eggs in a four hour period each day.