After pushing reset button,how long before water gets hot

SOURCE: hot water gets too hot trips off at tank then

It's time to replace your thermostat. Instead of maintaing the correct water temperature, the water is getting so hot it is acivating the safety circuit breaker. If this breaker were to fail, you could have a devastating explosion

SOURCE: Reliance 606 electric 40 gallon hot water heater

ECO reset button trips for several reasons.

1) Easiest thing without testing, is replace both thermostats.
http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html

2) If problem persists, then test elements.
http://waterheatertimer.org/How-to-test-water-heater-element.html

3) If problem continues, do full troubleshoot
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

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SOURCE: how long after pushing reset button,does water get hot

why did reset button need to be pushed. what caused it to trip. that may still be the issue.