I'm sorry, there is really no rule. It all depends on the context and what time scale you are thinking of. It is quite possible for a geologist to say "On a geological time scale, it was not so long ago that humans first evolved--no more than two million years ago." "Not long ago" and "not so long ago" mean "not long in comparison to other events we are thinking about."

I feel (and it is a feeling, it's not exact) that not long ago and not so long ago are referring to less than a 100 years. Not long ago, women were unable to vote in the U.S. Not so long ago, women were unable to vote in the U.S. But it's all contextual. When have you last had a tetanus vaccine? Not long ago. In this instance the speaker means a few years or less. The "so" is an emphasis word and doesn't change the meaning

Depends entirely on context.... minutes, days, years. To an old man, we first put a man on the moon not so long ago. To a child, the last time they ate may be seem long ago.

There is no exact number of days, but the situation does change how we use it. In most conversations, "It happened not so long ago," is used to describe something that happened between 1-6 months ago. We would not use it to describe something that happened last week or two weeks ago. Example: "I didn't take that class that long ago, but I've already forgotten everything I learned!" "Oh I went there not so long ago, they have the best cocktails!" If in regards to a major life event, it can be for a longer period of time. For example, "Her dad died not so long ago. She's still feeling very sad about it." If her dad died a year ago, this "not so long ago" could still be acceptable because we like to give people time to process major life events. Beyond a year, I wouldn't use "not so long ago."

They are just the same. I would say they are both simply synonyms of 'recently'.