For all three phases of the Dragons Challenge, both phases of the Standard, Rapid, Classic, Royal & Royal Pro Challenge, and the single-phase Classic, Knight, Knight Pro Challenge, the evaluation period will begin as soon as you purchase the challenge.
Please note that all challenges come with an unlimited expiry period, provided you place at least one trade every 30 days, failing which your account will get breached due to inactivity. To know more about the inactivity rule, please refer this article.