Duration: Spark duration determines how long spark projectiles will persist, and is increased by both skill duration passives and support.

Ricochets: Spark projectiles will bounce off of terrain obstructions and edges for as long as they persist. If a spark projectile strikes a mob, it will be absorbed unless it can pierce.

A single cast of Spark can hit an enemy once every 0.66 second (after patch 2.3). Keep in mind this is per cast, not per projectile. So without any increase in duration, a single cast can hit an enemy thrice with the base duration of 2 seconds. That is why projectile speed and increased duration can result in significant increases in real DPS.

When an enemy is hit with a Spark projectile, projectiles from the same cast will go through that enemy without damaging it, as if it wasn't there (until the enemy can be hit again with the same cast which happens after 0.66 second).

There is no guaranteed distance or time the projectiles will go straight, but they are guaranteed to go forward if there are no obstacles. After patch 2.3 Spark projectiles now destroy themselves after travelling 150 units from the character. It should be noted that it's entirely possible for infinite-duration sparks to bounce/circle around within that radius, and never leave it. In general, sparks will be more likely to stay within the radius in an enclosed space with walls to bounce off, and more likely to leave it in open areas.