How bad is it to drink a 2 liter bottle of soda in a day?

Severe tooth damage. Sodas are very commonly acidic in nature and it's chelating agents can soften enamel. It can rapidly cause the destruction of teeth. Do your best to minimize intake.
In one day? No big deal. On a regular basis, i'd give it a 7 on a scale of 0 to 10, with 10 being the worst.