Oral DXM Dosages http://www.erowid.org/chemicals/dxm/dxm_dose.shtml
Threshold unknown
Light 100 - 200 mg
Common 200 - 400 mg
Strong 300 - 600 mg
Heavy 600 - 1500 mg
Risk of Death 2,500 - 20,000 mg 1

Duration : 4 - 8 hours

Some users report that DXM dosages are affected significantly by bodyweight and lighter users should start with lower dosages. Because everyone reacts differently, it is a better idea to start at lower dosages and slowly work up rather than jumping in at a higher dose. Some people are very sensitive to the effects and may be overwhelmed by dosages that are minimal for others.

and i know nyquil doesnt make a product w/ only dxm in it so you shouldnt try and trip on it even though the amount you took is basically the recommended dose to control a cough.