The Underdark and Grymforge is what causes the bulk of the long rests.
I haven't played much of patch 8. Got to grove and stopped. In that game, still hadn't taken a single long rest. Used up my SRs and used Song if Rest. So it's coming soon. But it was certainly not hard to make it that far without resting once.
I'd say surface is in total like 4-6 LRs, and Underdark and Grymforge are like 6-8.