I think I can be more confident of the math for this one, another idealised case that doesn't account for differences from heat loss, substrate, premature switching off, etc.
Say the heater switches on when the temp drops to 25 degrees, off when it hits 26 so we're talking the time required for a 1 degree rise in temperature.
Your tank plus sump would have a volume of about 80 liters or 80,000 grams of water. That would take 80,000 calories to raise its temp by 1°C.
300 watts= roughly 258,000 calories per hour. So 80,000/258,000=.31 hours or about 22 minutes.
Hardly a strobe light turning staying on that length of time, and in reality once you throw in that it also has to counteract the cooling while heating it's going to be far longer.
Like I said, that would be a peak, the average rate would likely be a fraction of that.