How high is up or how long is a piece of string might be easier to answer, but...
All search engines strive towards the elusive goal of absolute relevancy so in an ideal situation when you typed in a query the very best page for your use would come up. But they have not come close to reaching that goal for many reasons including:
- Inability to interpret users request corectly
Multiple pages offering mostly the same content
Artificial techniques used to make irrelevant pages appear relevant = SPAM
So when google says(for example) don't use hidden text or we will penalize you for it, and you still see top ranking pages month after month which contain hidden text its not that google doesn't mean what it said or that the technology to detect hidden text is not avialalbe, but more likely that in the trade off of speed for accuracy thats a bit of that has not been automated yet. Its also why you have to take Public statements by SE reps with a grain of salt. When a google rep says "Yes we have the ability to detect hidden text" you have to think a bit and realize that having an ability and using it are often different things.
Now that doesn't mean that if you use hidden text you won't be penalixed for it, Google often runs a special campaign to root out one type of spam or another, and there are many webmasters who will find and report spam to google.
Bottom line the objective and desire to rid the index of spam is there, but the resources to do so may not yet be available.
More importantly you don't have to resort to such techniques to rank well.