Dan,

That is right! You will get at least 6 min if the full throttle current is 6 amps with a fully charged 600mah battery. In getting an esc, a speed 400 geared motor can require as much as 9.4 amps when using an 11x8 prop (it will also be putting out 15 ounces of thrust), so you want to get an esc that is capable of handling perhaps 12 to 15 amperes. A larger prop increases the load on the motor, so the motor works harder to carry the load. You can find this data on page 51 of the Hobby Lobby catalog - the GR3321/GR1718 is the speed 400 7.2v motor with a 4:1 gear ratio (not 2.5:1). That would be in case you wanted to up the power. Also note that the trade off is shorter motor run time - .6 divided by 9.4 equals .06hrs which is 3 min49 sec.

I might also mention that all batteries are not necassarily equal - some are better and some are worse, so the numbers could be slightly different.

Good luck