I looked on Erowid.org for you and it said to get to the First Plateau, it takes roughly 20-40 minutes. It also noted that 1200mg is a heavy dose, so you will probably feel it before the. However, I only "did" DXM once and found it boring and unpleasant, so my experience is merely academic.
1200 is a heavy dose. I wouldn't recommend it if it's your first time. How long it takes to kick in depends on what you're using. Cough syrup kicks in faster than gelcaps. Could vary as dramatically as 30 minutes to 3 hours.
lol i did it with 40 30mg mucinex pills i wouldint sugjest doing this cuz of all the gaunsefien i puke a little but over all its was a amazing intense fun trip:2thumbsup: