Using 5-7 drops of foundation per day:

Mini/Travel size foundation: 6 mL or 0.20 fl oz., can last up to ~1-2 weeks

Half-Ounce size foundation: 15 ml or 0.5 fl oz., can last up to ~1 month

Full Ounce size foundation: 30 ml or 1 fl oz., can last up to ~2 months