Usual Adult Dose for Cough for Robitussin DM are as follows:
Dose referred to by strength of dextromethorphan/guaifenesin (in mg) per unit of measurement:

10/100 per 5 mL: 10 mL every 4 hours, maximum 6 doses per day
10/187 per 5 mL: 10 mL every 4 hours, maximum 4 doses per day
10/200 per 5 mL: 10 mL every 4 hours, maximum 6 doses per day
20/400 per 5 mL: 5 mL every 4 hours, maximum 6 doses per day
20/200 per 10 mL: 10 mL every 4 hours, maximum 6 doses per day
20/200 per 20 mL: 20 mL every 4 hours, maximum 6 doses per day
20/400 per 20 mL: 20 mL every 4 hours, maximum 6 doses per day

Oral solids (tablets/caplets/capsule/gel caps):

10/200 per caplet: 1 or 2 every 4 hours, maximum 12 caplets per day
20/400 per tablet: 1 tablet every 4 hours, maximum 6 doses per day
30/600 per tablet: 1 to 2 tablets every 12 hours, maximum 4 tablets per day
60/1200 per tablet: 1 to 2 tablets every 12 hours, maximum 4 tablets per day

Doses of Robitussin-DM depend on patient age and other medical conditions the person may have.

Robitussin-DM contains two active ingredients, dextromethorphan and guaifenesin.

For full information on doses of dextromethorphan and guaifenesin see:
https://www.drugs.com/dosage/dextromethorphan-guaifenesin.html