How precise must a pilot be when flying a holding pattern?
Flying holds does not have to be rocket science and the holds do not have to be perfect ovals.  In fact when applying proper wind correction the pattern will not be a perfect oval.  There are two main parameters you need to meet to be safe and keep ATC happy when flying a holding pattern:


Maintain your altitude:  Sometimes an ATC controller will assign multiple aircraft to a published hold but assign different altitudes.  Any deviation of altitude could reduce separation to unsafe levels.
Stay on the cleared side of the hold as much as possible:  While there is some leeway if you cross over to the uncleared side (parallel entries typically do cross over to the uncleared side) you want to stay on the cleared side as much as possible.  This not only helps with separation of traffic and terrain but also helps keep you from drifting away from the fix especially if you are encountering winds aloft.


As far as your question concerning the time it takes to fly a holding pattern that is not that important as long as you are not drifting away from the fix.  Lets say you have a tailwind when flying towards the holding fix which will turn into a headwind when turning outbound.  The goal you want to obtain is 1 minute inbound legs.  But to maintain that you will need to increase you time on the outbound leg since you are flying into a headwind and losing ground speed.  That means your holding pattern will take longer to complete but ATC will not care about that.  ATC wants you to keep you altitude and not drift away from the fix.  Whatever you have to do to safely make that happen is fine with them.  

Question about 8 shaped oval:  I assume you are talking about if you accidentally turn into the uncleared side of the holding pattern and fly a figure 8 to get back on course and how to avoid that.  The rule of thumb is apply two or three times the amount of wind correction on the inbound course to the outbound course.  The reason this is done to account for the difference in turning radius when turning into the wind versus turning away from the wind.  The resulting pattern will not be a perfect oval but will look like the pattern below.  (Image borrowed from this math stack exchange post).



If you would like more info on what airspace ATC actually expects you to use for a hold click here and see wbeard52's answer.