As long as and so long as are conjunctions.
As long as
We use as long as to refer to the intended duration of a plan or idea, most commonly referring to the future. We always use the present simple to refer to the future after as long as:
We are very happy for you to stay at our house as long as you like.
I’ll remember that film as long as I live.
as long as I will live.
As long as and so long as
As long as or so long as also means ‘provided that’, ‘providing that’ or ‘on condition that’:
You are allowed to go as long as you let us know when you arrive.
So long as is a little more informal:
You can borrow the car so long as you don’t drive too fast.