Well, a big dexer baker I once knew told me "After a few hours you'll regain the ability to think."
Should depend on the dosage though...above 300mg and I would say you're in for a good four hours of being fucked up, then several more still feeling "out of it". This all depends on individual reactions as well though, a minority of the caucasian population (like 1 in 10 i believe) lack the enzyme used to metabolize DXM and thus for them the effects will probably be much stronger and last longer. Note that your pupils will be dilated and your eyes will probably get pretty red...then the whole robo-walk issue. So have a good six or so hours between onset and going home.