Like CR and SheFly say, its about YOUR personal best...

Other people are good reference to have as to how your imrovements are going
Sometimes other's time can be a good motivator.

I think the fastest TT time was in a Tour de France prologue - it was less than 10kms from memory and the guy (not Lance) averaged about 57kph!!! I'll post his name if I can remember it.

As for me - my first club TT about a year ago was 18km long and I averaged 27.7kph... my 25kTT a couple of weeks ago I fell just short of a 32kph average.
In terms of a TT thats a significant difference, though I still have a way to go before I am fast as some of the other women my age (42 in August).

One of the ways you can test your speed is to set yourself a TT on your own. Remember though, you time in a race under similar conditions will usually be faster than a time you have set on your own.

Enjoy your first TT... but be warned... if you like it, you'll be addicted