There is no maximum time limit for completing the challenge. You can take as much time as you need to reach the profit target. However, you must trade for a minimum of 4 trading days in each phase to ensure that your results are based on consistent trading performance, not one-off trades.
In other words, there is no countdown for the challenge duration but a countdown for inactivity does begin from the purchase date, so be sure to engage in some trading activity at least within 90 days.