When to go
Because Qatar has fairly consistent weather year round, all months are equally good to visit the Desert Safari dunes.
I think the morning is the best time to ride the ATVs because you can avoid the crowds and you'll have softer light. We had the dunes all to ourselves until about 10AM and then more people started to come.
Field tips
Access to the sand dunes is free, and most of the locals will bring their own, but If you want to rent an ATV there will be a cost. The cost varies and is determined by size, number of people and speed.
When taking photos, I felt like a 70-200mm lens was the most ideal for this view because I could really punch in and create the most depth while showing the various dunes in the background.
A long lens (like a 70-200mm lens) will allow you to capture tighter shots and a mid-range lens will give you more opportunity to shoot the landscape.
Action shots of a subject riding the ATV are what I wanted here, but you can also take photos of people standing just to the side or even walking through the dunes without an ATV.