When you start up a project with a budget, your project will remain open until the budget has been met. Once the budget has been met, it is not possible to link new challenges to your project. But the challengers already linked to your project, can continue to raise funds even if the budget has been met.
It makes more sense to create projects with an enddate in case of emergencies. Then there is no limitation as far as the budget concerns. You can raise as much money as you want to before a specific date. But note that challenges ending after the enddate can not link their challenge to your charity project.
Example: If you want to raise as much money before 01/05/2017, the challenger running the marathon on 15/07/2017 can not link his challenge to your project. As his challenge expires after your project.