Help with functions?

Yes, you can. It's called a debounce. A debounce is basically a variable that is usually set to false. When a player does something, you set it to true. However, you use a if statement so that the code will only execute when the variable equal false. Therefore, you can use this to prevent your code from running multiple times in a given time frame.

This is most commonly used in Touched events, so I'll make that my example.

local debounce = false
script.Parent.Touched:connect(function(hit)
    if debounce == false then --If debounce is false
        debounce = true --Set it to true, so that the code cannot run again until it equals false.
        print("You touched me!")
        wait(3) --After 3 seconds
        debounce = false --Set it to false so that the code can run again.
    end
end)

Use tick() to measure the time between your press and the last time the function ran.

NOTE: Not Tested. May not work.

waittime = 3 --How much time you want between each press.
requiredkey = "r" --The key you need to press for the function to run
local mouse = game.Players.LocalPlayer:GetMouse() --Get the mouse.

lastran = 0 --For use later

local function onKeyDown(key) --Key is what key the player pressed
    lastpressed = tick() --Last time someone pressed the button
    if string.lower(key) == string.lower(requiredkey) and lastpressed - lastran >= waittime then --Checks if player pressed the right button, and checks if there were 3 seconds between the last time the player pressed the button and the last time the function ran
        --Do what you want the button to do here
        lastran = tick() --Sets a variable for the last time the function ran
    end --End the if statement
end --End the function

mouse.KeyDown:connect(onKeyDown) --Connect the function to the mouse