HOW-LONG:
In American Sign Language (ASL), the sign for HOW-LONG is done by signing a single movement version of "HOW" and then smoothly transitioning into the sign for "LONG".The general sign for LONG is created by running your dominant index finger up your stationary, non-dominant forearm. This motion mimics the visual concept of using your arm as a measuring stick.
Extend your non-dominant arm out in front of you. The exact location isn't important -- just out in front of you in your general signing space. Keep your hand in a flat shape with your palm facing down towards the floor. It is okay though if your non-dominant hand is in more of a loose handshape since it just transitioned from the sign HOW.
Place the tip of your dominant index finger near the wrist of your non-dominant arm (on the top side).
Slide your index finger smoothly up the top of your forearm, moving from the wrist toward your elbow.
Non-Manual Marker (Facial Expression):
Because HOW-LONG is a "wh- question" (who, what, where, when, why, how long), when using this sign as an actual (non-rhetorical) question -- you should furrow your eyebrows and lean slightly forward while doing the sign.
Also see: HOW | LONG