How can I tell if a User is holding down a button?

One method could be to add a debounce and a while loop, for example when you press the button you change the debounce to true and when the button is released, make it false. Then all you have to do is use a while debounce == true do loop.

KeyHeld = false

function onKeyPress(inputObject,gameProcessed)
    if inputObject.KeyCode == Enum.KeyCode.V then
        KeyHeld = true
        while KeyHeld then
            --do stuff
            wait()
        end
    end
end

function onKeyRelease(inputObject,gameProcessed)
    if inputObject.KeyCode == Enum.KeyCode.V then
        KeyHeld = false
    end
end

game:GetService("UserInputService").InputBegan:connect(onKeyPress)
game:GetService("UserInputService").InputEnded:connect(onKeyRelease)

This is how I've been using controls for my games:

local tabDown = false
local input = game:GetService("UserInputService")
local player = game.Players.LocalPlayer

input.InputBegan:connect(function(k)
    local key = k.KeyCode
    if key == Enum.KeyCode.Tab then
        tabDown = true
    end
end)

input.InputEnded:connect(function(k)
    local key = k.KeyCode
    if key == Enum.KeyCode.Tab then
        tabDown = false
    end
end)

while wait() do
    if tabDown then
        print("Tab is down")
    else
        print("Tab is not down")
    end
end

It works very well for me. Of course in your case you would have to change the Enum for the key but other than that it should work.