Tapering

You're not really going to lose any stamina during a taper, it's about recovery to do the race with the correct inenstity. That said it all depends what you do during the taper and the affect on race day. 

In the Spring of 2006 USAT Triathlon Life mag there was a good article about training effects.  For the taper they listed 3 types of tapers:

1. Total rest - it was stated that this method results in about a 3% decrease in performance. Obviously this was not recommended.
   
2. Increase volume but lower intensity - stated that this results in a a 6% increase in performance.  Typically this is used for a C & B type event where you are in a build period before what would be your A race later on.  Example they provided was a 20% increase in volume.
   
3. Increase intensity but lower volume - This was stated to increase performance 22% and is used for your most important race. Example they provided was a 20% decrease in volume.

Taper length also depends on your current fitness, training, etc. I would have to say, however, that 10-14 days for an Oly seems a bit exceessive, as does 7-10 days for a sprint...

True, true, but lets also not forget to add age to that! That's what I do and seems to work the best for me, should have added the 'YRMV' phrase. I take longer to recover things like weekend warrior ball and even hangovers than I did years ago!

Again (my opinion) your not going to lose any stamina, that was his concern.  From what I've read doing any hard training wise up to 10 days out isn't going to be beneficial from a fitness point of view.