I've been taking DXM for 5 years.
After years of taking it I've built up an insane tolerance.
Your tolerance will never completely return to normal but after a few weeks to a month, you should be back down to a tolerable dose. Factors such as what and how much you've eaten, how hydrated you are, what other drugs you're on (prescription or otherwise), and whether you took dextromethorphan hydrobromide (robo, CCC's, etc) or polystirex (delsym or other "time release") also matter.
Do NOT keep doubling your dosage. Up it a little at a time, by maybe 30-90 mgs at a time. Doubling your dose is an easy way to take too much and have a very bad tripp.
I'm sure you've figured this all out by yourself by now but just in case, thought I'd fill you in.