When does depression leads to suicidal thoughts?
When depression is untreated it leads to suicide. And sometimes happy people also commit suicide. There are sign like giving away your precious things, saying you won't be a problem, loss of interest and isolation and etc. Suicidal thoughts are one of the main symptom of depression. Not all people with depression commit suicide.
Society needs to understand, depression is a real illness, it is not in person's head.
Suicidal ideation is one of the symptoms of depression that might appear after a couple of weeks of being depressed. It's not set in stone, but in my opinion it makes sense that the longer a person is living with depression the more likely it will be for them to develop suicidal thoughts.