As Pete points out, Heat is a good indicator. If you ever took Chemistry, its similar to those Super Saturation Enthalpy lab experiments.... as the compound gives off electricity it also generates heat, too much it breaks down internal bonds of the remaining paste. Early voltage drop is also a good indicator as well re battery health.
Trouble to is some batteries receive cooling which make them perform better but still past there limits. So keep within the math.
You're asking the question and we're turning it into a science lesson
Take a look back at the Battery List
that was posted earlier. It has a few vid demos of 25C batteries with this model.
Since you're flying stock, start looking to land as said around 3 min. Try not to peak the throttle at this point, just easy final circuits to home. 2 min if you are getting used to the model and need hard power to recover 2 or 3 botched approaches; one too many times and you may have a puffy battery.
And Pete as I pointed out I only use the 5800 30C for recon. They are too heavy to bother trying to use them 3D though they did do this fine the first few minute. Basic easy flying, testing etc because it has a longer flight time option where my level flight cruise drops to 2x27amp or about 5min. Otherwise its the GenAce 5300 30C or Genesis 5500 55C.