how to detect if player is holding a certain key for a certain time?

It should be local script because key detection is mainly used locally.

You could try something like this

local UIS = game:GetService("UserInputService")
UIS.InputBegan:connect(function(keycode)

if keycode.keyCode == Enum.KeyCode.w then -- Change w to the key you want
    local keydown = 0
    for rounds = 1,(5 * 4) do --change 5 to the amount of seconds you want
        wait(0.25)
        if keycode.keyCode == Enum.KeyCode.w then -- Change w to the key you want
            keydown = keydown + 1
        end
        if keydown ~= rounds then
            return
        end
    end
    if rounds == keydown then
        --Heres the script thats going to be executed after holding keydown
    end
end

I haven't tested if this works. And there might be a better way.