What is the adjective for long?

The most common adjective for "long" is long itself. However, there are other adjectives that can describe length depending on the context:

General length:

* extended

* prolonged

* extensive

* elongated

* lengthy

* sizable

* substantial

Specific to time:

* durable

* enduring

* permanent

* lasting

Specific to distance:

* distant

* remote

* far-reaching

* broad

Other possibilities:

* tall (for vertical length)

* wide (for horizontal length)

* deep (for depth)

The best adjective to use will depend on the specific situation and what you are trying to convey.