I'll use Website as long as it _______ free.

As long as
- We use as long as to refer to the intended duration of a plan or idea, most commonly referring to the future.
- We always use the present simple to refer to the future after as long as. 
Example: We are very happy for you to stay at our house as long as you like.

সুতরাং সঠিক বাক্যটি হবে: I'll use Website as long as it stays free.