When is a good time to Apply FE practicing Lua?

I have an above average knowledge about lua, but what I'm contemplating on is when a good time is to learn FilteringEnabled, or rather practice. I already know what what It does.

What I'm worried about is if I continue going the way I am without FE it could disrupt my flow sometime in the future.

tool = script.Parent
local char = script.Parent.Parent.Parent.Character

local en = true -- cooldown Animation
local sw = false
local on = 1


tool.Activated:connect(function()
	if en == false then return end
	en = false
	local char = tool.Parent
	local hum = char.Humanoid
	-- animation
	local anim = hum:LoadAnimation(script.Parent["Animation"..on])
	anim:Play(0.2,1,1)
	sw = true
	wait(0.5)
	sw = false
	on = on + 1
	if on >= 3 then
		on = 1
	end
	en = true
end)

local ten = true -- cooldown damage

tool.Handle.Touched:connect(function(hit)
	if sw == false then return end
	if ten == false then return end
	ten = false
	if hit and hit.Parent:findFirstChild("Humanoid")then
		hit.Parent.Humanoid:TakeDamage(15)
		wait(0.5)
		ten = true
	end
end)

Just a basic sword script ^

This is just an example of what I'm currently doing, and if I should simultaneously practice events etc, + FE combined.

The sooner, the better

You should learn FE as soon as you can. Your game will be in Experimental mode until then, and the more you learn about scripting w/o FE, the more you'll have to relearn to do with FE. I am currently learning how to use FE and it can get challenging to convert scripts to FE compatibility, but in the long run it will pay off by making your game a safer place to play in.