I only like DXM in light doses, Verry opiate like for me, couldnt imagine tripping off it to be fun....but to answer your question:
1 you should get Delsym, its only active ingredient is DXM ( read the botttle as I'm not sure which type)
2 IDK how many mg's you have, read the bottle and do the math and compare it to erowid and other online sources to figure out your dose.
3 some people process dxm differently and trip for much longer than "normal" people...but I think its pretty rare.
acitominophen(sp?) can FUCK up your body if you take too much...beware