Turtle is a buff monster. It gives you gold, additional shields for the team, as well a much faster level experience. In that case, getting Turtle is the most appropriate choice. If a team manages to get the Turtle once, twice, then three times, it's already confirmed the advantage for their team in that match. Of course, it will increase their win ratio in the match.
The turtle will appear when the 5th minute of the game comes. If you manage to get the first Turtle in that minute, the Turtle will come back again within 3 minutes. It happened before the 20th minute came. When the twentieth minute comes, the Turtle will be replaced by Lord.