It varies a lot depending on the person but by definition, symptoms have to last for at least 2 weeks. Untreated depression can actually go on for months; therefore, it's important to seek professional help when needed becase the goal of treatment is to end the symptoms as soon as possible.

Yes. In fact, people who have had depression are actually more prone in experiencing it again. Stastically, each episode of depression will increase your chances. After one recovery, there is a 50% chance of it returning, after 2 recoveries there's an 80% chance of it returning, and after 3 episodes it goes into a 90% chance.

No, and realistically we don't know how if there even is a way. It doesn't segregate out by whether you've had a lot of stressors or haven't. It's completely random, but a lot of stressors can certainly make it worse.

With a lot of research over the years, depression has become an illness that we can identify readily. It used to not even be considered a thing, but over the years with all the increasing evidence doctors have developed a good understanding of this illness, making it easier to diagnose, causing it to be more prevalent now.

Grief is going to be a series of negative emotions that can happen after a major loss and this is a completely natural human response. The emotions are going to be directly related to this loss and there aren't going to be all the symptoms of depression. Complicated grief can look a lot like a mood disorder and can sometimes end up transitioning into depression, but the key difference is grief won't have the entire feature set of depression.

Typically the main thing that you can do is just listen to them, allowing them to talk to someone. However, it's also important to recognize when they might need further help. Whether it's telling a parent or alerting a school counselor, making sure you let someone know can be extremely helpful to them.

Absolutely, any socioeconomic status, any nationality, anybody.