Both possible, depends on the whole sentence. Here an explanation I copied from another website:

When they are used in the comparison of physical lengths, use <as long as> for affirmative sentences and <so long as> for negative sentences.
This bridge is as long as the Golden Gate Bridge.
That bridge is not so long as the Golden Gate Bridge.

When <as (so) long as> is used as an adverbial conjunctive similar to <provided that ...>, you can use either, though <so long as> sounds somewhat more archaic than <as long as>.
As/so long as you stay in US, you can improve your English speaking skill.
I can read any book as/so long as it is written in English.