GoFundMe campaigns do not have any time limits or deadlines. You can keep your campaign active until you are ready to end your campaign. Find out how to do this here.
However, please note that you will have a deadline for setting up withdrawals for Personal campaigns. These deadlines vary by payment processor, but you will receive email reminders letting you know if your deadline is coming up and action is still needed! Setting up withdrawals will not end your campaign, and the total amount raised will not be changed on your campaign.