theres two methods i can think of off hand: first you could use a timeline to set the value which would allow a smooth transition, second you could use a timer if you wanted a more incremental fill.both methods are fairly simple. with the timeline you could just make a curve that goes from 0-1 over a given time and you can make it go at a linear or non linear fill speed as desired. if you need more info on these methods let me know.
you may also look into the interp nodes but as its not something i use alot i cant help with that.