0 to 1 is a percentage, however using like you are results in it never getting to 1, or 100%
You have to realise that current value is the only value that's changing.
In order for it to work as a percent you would have the start value, to the target value.
lerp(startvalue,targetvalue,0.1)
lerp(startvalue,targetvalue,0.5)
lerp(startvalue,targetvalue,1)