Pressurized fresh water flush requires a minumum flow RATE of of 3gpm at 5 psi...but that only describes the amount of water pressure needed, not the rate.

Whether the toilet is a sea water model or a pressurized model, the actual amount of flush water used would depend on how long you leave your finger on the button...according to my sources at Raritan, the average is about a liter for a sea water toilet, about 30% less for the pressurized version.

The above only applies to the single flush button SeaEra, btw.