I already have a aquarium doser running with ESP8266 NodeMCU Dev. Kit.
The actual time to start each doso is got from the RTC widget.
I already tested that during a device internet disconnection, the exact hour, minute and seconds will be still available on the device for a long time.
My question is: assuming that the device not try to reboot, how long the time calculation will keep ok during an internet fail?
Any experience on this?
You’ll normally find that the drift is less than a second per day.
That’s means if your internet is down for 2 months your dosing time will be out by about 1 minute.
If my internet was down for 2 months it would probably mean that we were in the beginning of a nuclear winter and those fish would have ended-up on the dinner plate long ago 
In reality, I guess your biggest issue is your NodeMCU rebooting in this situation, so some sort of UPS would probably be a good idea.
Pete.
jajajaja.
The greatest answer ever!
Thank you my friend, you made my day.