In high season the local guides go 3 times a week out there. Wednesdays, Saturdays and Sundays and they start walking at 11am. If you want to visit for example at sunset then you need to ask them if that is possible. You have another option though.
I do special trips all year round with normal tourists and photographers. This hike takes about 5 - 6 hours in all. Important that you take food and water with you.
Bonus tip: If you don’t have much time on your hands to hike that long it is also possible to go by boat. Once we go by boat it takes only 15 minutes to arrive there. It is also possible to go sightseeing between the sea stack Drangarnir. Normally the sightseeing trip takes about 2 - 3 hours. If you want to know more about this, just contact me and I will be happy to help you out :)