Actually it depends on several factors that SL carefully programmed into the game code. As a quake "receiver", if I am leading the race it keeps going until it gets me, no matter how far. This can be up to 3 laps, if necessary. If I am in a close second place, it goes just far enough to get me but not the leader, dying out immediately after bringing me to a half and only warming the leader`s tailpipe. Quakes also adjust their speed to make sure they get me just as I am crossing track splits, ensuring that I plummet to my doom and go from first to last place. As a quake "giver" different rules apply. Now, these track splits act as barriers that quakes cannot cross. If they do not hit these barriers, quakes travel about 3 seconds forward, at most, and usually they trigger a shield pick up for the race leader to make sure he isn`t hurt. That`s how it works.

Actually I think it`s just under six seconds, not as much as half a lap, unless you`re Wellington in which case that is about half a lap. I think they were made to travel slightly further in HD to give chasers more of a chance, but I don`t remember if this came from SL or not.