Using 1000x WS2811 leds and 5v 60amp Power supply. When do I need to re-inject power?
Voltage droop is a result of cable resistance over large runs. Resistance adds up the longer and thinner the cable. And as your current load increases, the voltage you see at one end of the cable will decrease. Basic ohms law. In this case you will see the leds down the line look dimmer and dimmer until they stop lighting up at all.

You reinject power every few meters because you want to bypass the resistance of the cable to offer a path of lower resistance and keep the leds looking even.

Keep in mind the 60 leds or so statement is for basic 12V strips, not the smart 5V strips. Reinject power as you see fit (turn all the leds to full brightness white and eyeball it)