how do I make it so you have to told the Key for a certain period of time?

Hello, BookOfTragedy!

The script below should help you out. This script is just like yours but I've changed some stuff. Everything that is changed has been explained in the script.

local PlayerService = game:GetService("Players")
local UserInputService = game:GetService("UserInputService") -- Using UserInputService instead of mouse.

local Client = PlayerService.LocalPlayer

local Key = "M" -- The ke the person has to hold

local Holding = false -- Detects when the key is being held
local ExpectedHold = 3 -- How long you want the player to be hold the key for
local HoldingTime = 0 -- The amount of seconds they holded for

local Event = game.ReplicatedStorage:WaitForChild("Use")

UserInputService.InputBegan:Connect(function(Input, GPE)
    if not GPE then
        if Input.KeyCode == Enum.KeyCode[Key] then -- When they start to press the key this runs
            if not Holding then -- if they arent already holding then it continues
                Holding = true -- Makes holding true/script now knows they're holding the key

                while Holding do wait(1) -- Basically while holding == true the code after this will run every second
                    if HoldingTime < ExpectedHold then
                        HoldingTime = HoldingTime + 1 -- This adds to how long they holder the key for
                    end
                end
            end
        end
    end
end)


UserInputService.InputEnded:Connect(function(Input, GPE) -- runs when they stop holding a key
    if not GPE then
        if Input.KeyCode == Enum.KeyCode[Key] then
            if HoldingTime >= ExpectedHold then -- if they holding the key for the right amount of time
                Event:FireServer() -- fires the remote if they did
            else
                print(Client.Name..' holded '..Key..' for '..HoldingTime..' second(s).')
                HoldingTime = 0 -- sets the time back to 0
            end
        end
    end
end)

I would recommend using UIS over KeyDown as KeyDown is deprecated. UIS Docs. For this problem, I would use the InputBegan event and IsKeyDown function. You would use InputBegan to change the holding variable to true if the key m is pressed and you would use IsKeyDown to see if m has ever been let go before the 3 seconds is up. (You can find all of this in the UIS Docs). Reply to this if you have more questions.