How do I know that the project I'm supposed to join is a long term project?

Please go through with project planning or contract.

short-term projects take a few hours, days or weeks to complete. Long-term projects take months or even years to finish. Typically, companies require more documentation and infrastructure for longer-term efforts.

Decisions about project governance usually depend on budget, resources required, business impact, and the scope the project. Effective project managers start by assessing the project need and determining how much time is needed to meet the desired outcomes