Golden circle is one of the most travelled routes in Iceland. Due to proximity of Reykjavik it Can be perfect and easy trip for one day. (Perfect for long layover in Iceland)
Timewise we are looking at 3.5h of total driving plus all the time you will spend at locations - So usually total should be 7-8 hours.
How to get around
You have two options Rental car or Tour Bus:
TOURS
With tours you dont have to plan Anything they will drive you around and give you limited time at each location - but some people love it So if its for you Here are three best Golden Circle tours in my opinion:
Golden Circle — Platinum Tour | Small group
Golden Circle, Secret Lagoon & Kerið Volcanic Crater Tour
DT 16 Midnight Sun Golden Circle
Rental Car
Thats the best way to travel around Iceland in general - you have Freedom to stay at locations as long as you want and you Can explore side attractions.
Best time?
Whenever you are in Iceland! Obviously summer months will give more time as it is almost 24hr daylight :)