You definitely need a timer--you cant survive without one.
One rule of thumb is one complete turnover of your pool water through the filter once a day. Most cartridge filters are 10 micron in size, and 6-8 hours run time a day should be good for starters.
16'x30' =480 sq./ft.
Avg. depth of 3'x8' water is 5.5'
7.5gallons to a cubic foot
480sq/ft X 5.5 depth X 7.5 gallons= 19,800 Gallons.

Good Luck