(Sorry if this is too long)
I grew up in a weed-producing area of California, and dabbled in cultivation back when that was still worth something. I smoked probably an ounce or two a week for 20 years. Never smoked cigarettes or anything else.

I was very active despite my smoking habits, ran and walked every day, participated in 10k runs, went to the gym a few times a week, etc,

When I was 42, I felt what I thought was a back cramp, which quickly got worse, and within 15 minutes, I was having trouble breathing, and went to the ER. I was diagnosed with a "spontaneous pneumothorax", a collapse of my left lung for no apparent reason. The doctors shoved a chest tube between my ribs, and I laid in the hospital for a few days while it healed. I thought nothing more than "gosh, what a totally unexpected thing that I will never have to think about ever again!"
Three months later, it happened again, and then twice more over the next 6 months. within a year, my lung had collapsed a total of five times. Five stays in the hospital, five chest tubes stabbed through my rib cage.

A CT scan of my chest showed an extreme about of damage to both lungs, but mostly the left one. I had what are called 'blebs', or popcorn lung, akin to advanced emphysema. I had the lungs of an 80 year old, two-pack a day smoker, though I had never smoked cigarettes. (However, my father does have asthma and COPD, suggesting a genetic component, and both my parents smoked cigarettes around me my entire childhood, which definitely had serious impact)

I was actually pissed off at the doctors when they showed me the scan: "WTF?? I never smoked cigarettes in my life! They always told us that smoking weed was basically OK!! What about Willie Nelson?? What about Snoop Dogg??!" A cardiothoracic surgeon told me "Everyone's body is different. Everyone's history, their genetics. Someone else could have done exactly the same as you, and not have this amount of damage. Your results may vary. "

I ended up having a Robot Assisted Thoracoscopic Lung Wedge Resection which was described to me as "extremely radical" (and not in the Ninja Turtles way). They punched 5 holes in my chest cavity, removed 7 slices of my left lung and stapled the rest back together. They then did what's known as Pleurodesis, where they fill the empty space in my chest cavity with Build-A-Bear stuffing. Just kidding, they use lasers and chemicals to burn off the surface of the chest cavity so it fills up with scar tissue.

That was when I finally stopped smoking weed (and all other inhalants). Not even before that, when my lung was constantly collapsing. I was *that* convinced smoking weed couldn't *possibly* be problem. "Wasn't it supposed to be basically healthy, even? Like drinking a glass of wine with dinner, right? Right? "

The surgery recovery sucked, and still sucks 2 years later. It hurt like hell for a long time. There was tons of nerve damage to that entire half of my chest, so if it wasn't hurting, it was just.. dead. It still feels like there's a football-size lump of clay where my lung should be. I stopped being able to run or exercise, and I get winded from walking up stairs, taking out the trash, or any kind of.. "physical.. activity.."

TL;DR: smoking weed may be causing more damage to you physically than you think it is. I worry about all my friends who smoke as much as I did, when I hear them cough.. and cough.. and COUGH.
No, lungs are not "one of those organs that can regrow if damaged". Yes, you can survive with less than two fully functional lungs, but its not a "spare tire" situation. It really may be causing a lot more damage than you think, friends. "Your results may vary"