Are your projects just two-week projects for the client? Or, if they're longer, do you plan out all the sprints in advance (which isn't really Scrum best practi

Are your projects just two-week projects for the client? Or, if they're longer, do you plan out all the sprints in advance (which isn't really Scrum best practices as far as I understand?). I mean if you have a larger project (larger than two weeks) and you just plan for the next two weeks (as Scrum tells you to do, as far as I understand) how do you know that if you deliver this sprint you'll be able to deliver the project?