How much hours will new iFixit battery give?

Batteries are rated by watt hour (Wh) or milli Amp hour (mAh). In this case your systems battery and the replacement battery are the same 77.5 Wh/5200 mAh. It will run just as long as it did originally.

Think of it this way your cars gas tank volume is the same across all of the same model cars. How much gas it holds depends on how full it is. The gas’s energy value in truth is the same across all makers of gas! So Shell, Gulf or Mobile makes no true difference, as much as they try to imply it does.