There is No Time Restriction for completing the Express Challenge. You have the freedom to take as much time as you need to complete the Challenge.
But you have to maintain 10 minimum trading days on each trading cycle. For this, you only need to take a minimum of 10 individual and separate trades each month, and you must enter a new trade on at least 10 separate days in a month. Your trading days do not need to be consecutive and to make things even easier, there is no maximum number of trades you need to take each month.
Now, if the 10-day trading minimum is not met, you will not be disbursed any profits at the end of your trading cycle, and FundedNext will also reset your account to the starting balance. FundedNext would not reset your account to the starting balance if you were not in profit at the end of the trading cycle (4 trading weeks as one trading cycle). Only executed trades count towards your trading days.