Problem making a script verify a value constantly?

You are storing a value from IntValue into the Bob variable. That variable will never change unless you change it yourself. To make the variable change with the value, you have to store the IntValue itself, and then get the Value every time you need it, like this:

Bob = script.Parent.Bob

Dmg = 0.5*Bob.Value

wait(2) -- If Bob's Value changes in these 2 seconds, the variable below will have a different value than the variable above

Dmg = 0.5*Bob.Value

If you need any more help, make sure to leave a comment.

EDIT

If you want the Dmg variable to change every time the Bobs Value changes, you can bind an event, like this:

Bob = script.Parent.Bob
Dmg = 0.5*Bob.Value

Bob.Changed:connect(function(dmg)
    Dmg = 0.5 * dmg
end)

That Changed you see up there is an event that every object has, it listens for any changes in the object's properties. IntValue object has a Changed event which passes the new value (I called it dmg) to the function bound to it.