This should do it.
If you want saving that’s an entirely different story. Ask if you want help with that.
game.Players.PlayerAdded:Connect(function(Player)
local Leaderstats = Instance.new("Folder")
Leaderstats.Name = "leaderstats"
Leaderstats.Parent = Player
local Level = Instance.new("NumberValue")
Level.Name = "Level"
Level.Parent = Leaderstats
repeat
if Level.Value < 100 then
Level.Value += 1
end
wait(1500)
until Player == nil
end)