How to make a KeyDown function, but you have to hold down the button for a certain time ?

Hey there! I'll walk you through this with sample code and explanations. I'll assume you know how for loops and RenderStepped works as well as activating functions through events. I'll walk you through UserInputService, as KeyDown is deprecated!

First thing you're going to want to do:

Create a variable that is set to true or false depending on whether or not "E" is being pressed (I suggest calling it "epressed" or something like that)

Oh, and I also suggest using UserInputService, as KeyDown is deprecated. Here's an example of how you'd set that up:

game:GetService("UserInputService").InputBegan:connect(function(input)
	if input.KeyCode == Enum.KeyCode.E then
		epressed = true
	end
end)

game:GetService("UserInputService").InputEnded:connect(function(input)
	if input.KeyCode == Enum.KeyCode.E then
		epressed = false
	end
end)

Basically, InputBegan marks a key press, while InputEnded marks the end of a keypress. Now, when epressed == true, you want another variable that's basically a counter for how long you've held down the key. Name it ecounter or something like that. Set ecounter = 0 by default.

In a separate function that you should write above the UserInputService functions, write a for loop that counts from 1 to however long you want to wait. Sample code:

epressed = false
ecounter = 0

function countE()
	for i = 1,3,1 do
		if epressed == true then
			ecounter = i
			print(ecounter)
		end
		if epressed == false then
			break
		end
		if ecounter == 3 then
			--Do whatever you wanted to accomplish here! E has been pressed for the amount of time you want!
		end
			wait(1)
	end
end

game:GetService("UserInputService").InputBegan:connect(function(input) --E is pressed
	if input.KeyCode == Enum.KeyCode.E then
		epressed = true
		spawn(countE) --Function countE is run in a new thread. You don't have to do this, but I did in case restarting the function each time messed with anything.
	end
end)

game:GetService("UserInputService").InputEnded:connect(function(input) --E is releaseed
	if input.KeyCode == Enum.KeyCode.E then
		epressed = false
	end
end)

Pretty simple! It only counts up if E is being pressed down, and if not, it breaks out of the for loop! We also check if ecounter has actually reached the amount of time we want (in this example, 3 seconds) and within that condition you can do whatever you wanted the script to do once you pressed E for the desired amount of t ime. Now, in a RenderStepped event function using RunService, I made it constantly set ecounter = 0 while E is not being pressed. Using RenderStepped makes it accurate up to each frame that's rendered. Sample:

game:GetService("RunService").RenderStepped:connect(function()
	if epressed == false then
		ecounter = 0
	end
end)

Here's the whole sample script in action!

epressed = false
ecounter = 0

function countE()
	for i = 1,3,1 do
		if epressed == true then
			ecounter = i
			print(ecounter)
		end
		if epressed == false then
			break
		end
		if ecounter == 3 then
			--Do whatever you wanted to accomplish here! E has been pressed for the amount of time you want!
		end
			wait(1)
	end
end

game:GetService("UserInputService").InputBegan:connect(function(input)
	if input.KeyCode == Enum.KeyCode.E then
		epressed = true
		spawn(countE)
	end
end)

game:GetService("UserInputService").InputEnded:connect(function(input)
	if input.KeyCode == Enum.KeyCode.E then
		epressed = false
	end
end)

game:GetService("RunService").RenderStepped:connect(function()
	if epressed == false then
		ecounter = 0
	end
end)

This is probably not the most elegant solution in the world, but it definitely works. I spent ~20 minutes writing this to make sure I didn't leave anything out. I wrote this hoping that you and people looking up this question in the future could follow along easily! If you have any more questions, please let me know. If anybody thinks I should revise this answer, tell me! Thanks! :)