The time aspect of this calculation would be hard to implement since the rate that you kill thugs varies with individualized factors like gear, stats, how crowded, etc.
One way it could work is if the calculator was implemented into the game engine itself. Could be pretty cool to have a count-down somewhere on the screen which extrapolates your xp rate in order to determine how long until your next level. Sounds a little too flashy in my opinion though and it would be hard to tame in cases where multiple skills are being trained like superheating ores as you mine them.
My suggestion is just figure out your xp rate by typing ::info and going to RSC wiki and searching how much xp is gained from each thug/fish/ore.