I depends on what type of shower head you have. Most newer water saving shower head have a flow of 2 gallons per minute.
Approximately 2 gallons of water per minute, according to usgs.gov.
That may depnd on your shower fitting, water pressure, water tempreature and so on. A new New Zealand designed shower head called the Methven Satin Jet is proving effective at 7.6 litre per minute....
wterflow flow in gallon per minute
1 gpm