[Vlad]

You have to use a Frame that you put a OnUpdate script to do the calling for you.

For example;

local f = CreateFrame("Frame")
local count = 0
f:SetScript("OnUpdate", function(self, elapsed)
  count = count + elapsed
  if count >= 100 then
    count = 0 -- reset to we wait another 100ms before printing
    print("We do this each 100ms.")
  end
end)

There are many other ways too, using the animation system and even the lua threading I reckon, but I am not so god with threading in lua. You can also look up the widget API here: http://wowprogramming.com/docs