As long as indicates the planned duration (of something) or a condition. "You can stay with us as long as you like." "As long as" indicates that the idea of "you can stay with us" depends on the preference of the person that is shown as "you like" in this context. In other words, the person is given the permission to stay and the choice to choose how long they will stay, which is up to the person, but there are social factors involved because the phrase doesn't mean forever and the person who gives you the permission expects you to leave at some point in time. So it's a hospitable way of saying, "until you've decided on something better for yourself or to leave for other reasons, you have a place to stay at, which is with us, and we know you'll leave, but we want to give you a safe place and for you to not have to worry about accommodation until the time comes that you'll leave". "As long as you keep your promise, I won't be mad." Condition: if you keep your promise, I won't be mad This is also a good alternative to the sentence in which you substitute "as long as" with "if", but, to me, the usage of "as long as" feels more formal and somewhat authoritative compared to "if". So long as also indicates a condition, and I read that so long as is considered a bit more informal. "So long as you keep your promise, I won't be mad." Can also be substituted with "if" with the same difference in usage stated above (according to my own feeling). Personally, when it comes to the indication of conditions, I see "as long as" more often than "so long as".

As a native speaker, I use them interchangeably. They mean the same to me. I tend to prefer "as long as", but I believe this is simply a personal preference. I've lived in both the midwest and southeast of the United States, and heard both interchangeably.

They are interchangeable. "as long as" is a little more formal.

I would certainly agree with Michael. Both are fine but as long as is more formal ... I would say also a bit more common, in my experience