A good, general amount is 15-25 gallons (57-95 L) of water per shower. For reference, a 8-minute shower with a showerhead that uses 2.5 gallons per minute (9.5 L per minute) uses 20 gallons (76 L) of water. Find your shower's water flow rate!
Of course, the exact number is up to you, depending on the rate you're charged on your water bill (soon, Bathtimer will allow you to track water costs as well as set goals for water usage and cost)!