I cant really tell exactly.... but i would say ive been on for 2 years... But ive had sc for longer!

ADDITION:
o yea i forgot i stoped for about 6 or 7 months but... ah, wtever, i think i joined battle.net earlier then i was thinking... so... 2 years!
VOTE!!! I've been on 4 years plus. I did stop for about 1-1.5 years once because it was boring. But then for some reason I started to like it again and learnt map making.
97-99 Was addicted
99-01 Diablo 2
21-05 Got re-addicted
I'm in 10th grade now, been on b.net since about 5th or 6th grade when one of my friends told me about Starcraft.
I've been on for 4+ years, started in 4th grade just meleeing and stuff, then found out what UMS were and decided to make make about 3 years ago. But I took some lengthy breaks in between.
'01 - '03 = Complete SC
'03 - '04 = Diablo 2, pretty long time, probably 1½ years or so
'04 - '05 = Back into SC and map-making.
'99 = StarCraft
'00 = StarCraft/Diablo 2
'01 = Diablo 2
'02 = StarCraft/Diablo 2
'03 = Diablo 2
'04 = StarCraft/Diablo 2
'05 = StarCraft
I should get new games.

Ah lets see, I got Starcraft whenever, probably 4 years ago or something because my friends got it, played for a while, stopped, and went back to playing it again

Only one year... A half year with a

slow modem and a half year with a nice broadband

Let's see um... um... um... I joined SEN 8 months ago after being on Bnet for 1-2 years so... Dunno. Something between 2 and 3 years I suppose. Fells like a hell of a lot longer.
Hmm I guess this is..
2000th post

it feels longer for me too.... i asked how long weve had starcraft to my bro and he said 3 years and im like... "dude, its been 6 years!!!" Weird...
i got sc brood war in 1999, so 6 years for me
I think it's a lil less than 4 years, but more than 3,5...
Been playing SC since 5 years. Bnet 4. Eh.
Over 5 years now. I started making maps in the summer of 2000, then got my first famous one in december of 2000. Since then, i started churning out maps like crazy

wow, just realized that it has been that long..... like 4 years + for me.... dont remebr month or anything... but wow, it really has been that long.... weird
If i never asked my brother how long weve had sc... well... I woulda said... 4 or 5 years.
It feels like ive had it forevr.
under a month but ive had sc a long time
All I know is that I've had SC since I was 9.
somewhere between 3-5 years i plaed the demo i liked it and i got sc and bw for $70
2 years and around 6 months but not in a row. i Played back in the old days on b.net when sc was new then i quit to play other games(basicly only blizzard ones) and then started again.
'01 - '03 SC + Bits of D2
'03 - '04 WC3 and SC
'04 - '05 SC + Bits of D2
You can see a pattern

2000 got sc played for about 3 months got the net played on battle.net till 2002 got un-addicted for about 2 months and since then always been playing it, so 5 years.