I can't seem to get the logic down correctly so I need some help

Lets say I have a progress bar with a width of 300px. When I set the width/scaleX to zero, how can I increase the width/scaleX based on duration. So if I want the entire load progress to last three minutes, it will take the load bar three minutes to go from zero to either its maximum width (300px) or 100% scaleX?
PS: Using "width" or "scaleX" does not matter.
Thanks

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