How old is old gas

I have a 2008 8hp 4 stroke tohatsu that is used for pushing my sailboat. The engine runs for maybe 30 minutes when we are motoring out and 30 minutes when we are motoring back a few hours later. Obviously we don't use much fuel.
The tank sits in the cockpit of the boat, with the vent open. We don't have a vented locker for the tank, and I don't want to keep it in an unvented locker where fumes can accumulate.
Am I creating problems? Should I take the tank home and store in my garage each trip?
How long should I keep gas in the tank before I discard it? Where can old fuel be safely discarded?

I have the same situation on my sailboat only with a 3.5 four stroke.
First close that vent when not in use as the ethanol will absorb moisture
in the air and lead to carb troubles.
For my use I have gone to a small 1 gallon approved gas can and I
Refill when it gets 3/4 empty.
At the end of the week we empty any gas left over into our car
and refill on the next day of sailing.

The "rule of thumb" for modern E-10 fuel is to use it up within 30 days. That's for vented tanks, and modern, EPA-rated small motors, such as the MFS8A3. More important, though, is to run the carb dry if the motor is going to sit a week or more between uses, to reduce the possibility of varnishing in the small carb passages. We are having limited success using good fuel stabilizer, such as K-100 in our tanks. After a month or so, we dump the fuel into the car to run it out.

Since your 3-gallon tank is alternately exposed to sun/night, you can't reasonably keep the vent closed to reduce air exchange -- because the tank will swell up like a throw pillow in the hot sun. And, if it rains on your tank with the vent open, you risk getting rainwater in the fuel. So... if you don't have a good storage location aboard, it wouldn't hurt to take the tank home to the garage between sails.