To power an LED from push button trigger for 5 seconds I'm testing the below code, but there is no delay
Of course not. You need to record WHEN an event occurred, and later test whether that even was long enough ago to warrant having another event occur.
currentTime is a lousy name for when the LED was turned on.
loopTime is only set in setup, and is a meaningless name.
Try using some names that mean something, like onTime for when the LED was turned on. Ditch loopTime, which is completely meaningless.
You can determine that the LED is on because onTime will be non-zero. So, the else part is not needed.
After the if(digitalRead(8) == LOW) block, have another if statement:
if(onTime > 0 && millis() - onTime > 5000)
In that block, turn the LED off and set onTime to 0.