How do I make all the players have inf. health for a limited time?

What I mean by that is; when the game starts, everyone will spawn in a lobby. They will be in the lobby for a specific time (27 seconds), and then after 27 seconds I want them to have 100 hp. Please help! Thanks!

This is a easy script.

wait(.2) -- Making sure the character loads.
local ply = game.Players.LocalPlayer -- Getting the player
local char = ply.Character -- Getting the character. You could also do ply:WaitForChild("Character")
local hum = char:FindFirstChild("Humanoid") -- Getting the humanoid
hum.MaxHealth = math.huge -- Giving inf maxhealth
hum.Health = hum.MaxHealth -- Giving inf health
wait(27)
hum.MaxHealth = 100 -- Reverting health to 100

There you go ;)

This should do it, but you will have to do the part where when they enter the lobby it works

local Player = game:GetService'Players'.LocalPlayer
local Character = Player.Character or Player.CharacterAdded:Wait()
local Human = Character:WaitForChild("Humanoid")

Human.MaxHealth and Human.Health = 999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999 -- This is INF
wait(27)
Human.MaxHealth and Human.Health = 100