Paco2004 wrote:

Hello Tim I too am a mere English learner, but if you don't mind, let me try to answer. We use <as (so) long as> in two ways. 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. paco

Native speaker, but I have nothing to back me up.  My tendancy is to use one in the affirmative, the other in the negative:  "You can stay here as long as you want"; "You can't stay here so long as you continue smoking".  I have no idea where this distinction came from, and I don't remember studying it.  But it's how I use them; am really interested in hearing from other anglophones to get their take on the question!

Uuum, Clive, but still I want to ask why? A part of the entry 'long' in OED is as follows:

Long 1.b.
In the comparative and superlative, or preceded by advs. of comparison (as, how, so, thus, too, etc.), the adv. indicates amount of relative duration. So (or as) long as: often nearly equivalent to 'provided that', 'if only'. Also, long as, ellipt. for so (or as) long as.

c900 Baeda's Hist. iv. ***. (Schipper) 496 Ic..the..aetywde..hu lange thu on hreowe awunian sceole.  971 Blickl. Hom. 169 Swa lange swa ġe ðisdydon ðara anum ðe on me ġelyfdon.  a1225 Leg. Kath. 1816 To longe we habbeð idriuen ure dusischipes.  c1375 Sc. Leg. Saints vii. (Jacobus Minor) 623 Ay the langare he sat sa, the mare grew his sorow & va.  c1400 Lanfranc's Cirurg. 37 If that a wounde hath be to longe in the eir open thanne [etc.].  1433 Rolls of Parlt. IV. 424/1 Whiles and as longe as hit is or shall be soo.  c1500 Melusine lv. 331 So long rode geffray that he came to the Castel.  1513 More in Grafton Chron. (1568) II. 775 The Cardinall perceyved that the Queene waxed ever the longer the farther of.  c1560 A. Scott Poems (S.T.S.) xix. 13 How lang sall I this lyfe inleid.  1567 Gude & Godlie Ball. (S.T.S.) 27 Als lang as I leue on this eird.  1568 Tilney Disc. Mariage Cviijb, I have alreadie troubled them to long.  1590 Spenser F.Q. ii. viii. 28 The guilt, which if he liued had thus long, His life for dew reuenge should deare abye.  1631 Gouge God's Arrows iii. lxv. 304 A liquour ..which kept them from rotting, and made them last the longer.  1642 J.Shute Sarah & Hagar (1649) 171 Absalon ..kept his wrath so long; until it burst out into blood.  c1680 Beveridge Serm. (1729) I. 68 So long as there are devils in hell.  1715 Atterbury On Matt. ***. 25 in Serm. (1734) I. 127 Thus long have they [Jews] been no Nation.  1732 Berkeley Alciphr. ii. §20 The world+always will be the same, as long as men are men.  1776 Trial of Nundocomar 29/2 How long did you live with Sielabut at Delhi?  1807 Wordsworth To Small Celandine in Poems I. 22 Long as there's a sun that sets Primroses will have their glory.  1825 Thirlwall Lett. (1881) 85 To cling to your profession as long as you can.  1834 Southey Lett. (1856) IV. 391 God has mercifully supported me thus long.  1846 Browning Lost Mistress v, I will hold your hand but as long as all may, Or so very little longer.  1863 H.Cox Instit. iii. ix. 730 One-third who have been longest in office retire annually.  1870 Morris Earthly Par. I. i. 394 She stood so long that she forgot to weep.  1887 L.Carroll Game of Logic Pref., Is there any great harm in that, so long as you get plenty of amusement?  1938 G.Greene Brighton Rock i. i. 22 'It's all right,' he said, 'long as you are here'.