How much time does does a person sleep on 20 ml propofol?

Completely: asleep but start to wear off in few minutes

Varies a lot!: Patients who are already on sedative medications (Valium, Xanax) or who consume alcohol tend to metabolize meds faster. Youth and size also make a difference--the larger you are, the less effect a given quantity of med will have. Older patients take longer to recover. 20 ml or 200 mg of Propofol will induce sleep in almost every adult, but how long it lasts can vary a lot.

Variable: As the others said it is variable depending no your weight, medications, any competing sedatives, intoxication, stimulants etc. That dose will induce unconsciousness in the majority of persons and probably last around 10-15 minutes. Again this isn't sleep it is controlled unconsciousness