Depends on your SSRI. Prozac/fluoxetine has a really long half life, complicated by active metabolites and a tendency to inhibit it's own metabolism. I honestly wouldn't like to hazard a guess, and it depends how long you've been on it, but a quick glance at wikipedia tells me complete excretion could take several weeks.
Stopping your fluoxetine for some DXM really doesn't seem worth it, sorry sdaffer..