It's a time-based demo, so you get to play the full game. I believe it is 8 hours, which is more than enough to beat the entire campaign, or play a lot of ladder.
Or just make and netcode with a built-in DDOS attack system with a "deltree c: d: e: f: g:" -command in it that attacks every single drophacker when they execute it. izi pizi
with my copy of SC2 i got a 'Guest Pass'. it works for 7 hours of game time or 14 days from activation (whichever comes first). is this what you need? i am not sure if it works for both SP and MP, but i've been told it works perfectly for both.