As fast as you can.
But seriously, unless you intend to only compete in time trials, measures of speed aren't so relevant. A race is about tactical cunning as well as raw strength. Bike handling, hill climbing and so on also pay a part. A stronger rider can be beaten by a sly one who follows wheels.
If you want to keep a measure on how your speed is coming on, join a club that does evening TT's every week. Most clubs in the UK do this all summer. Then you can see if you can do the same course faster, in various conditions. The weather and the course have an effect though, as does the type of bike you are on. For example I started doing TT's this year, and did a 10 in 27:30. My best time, later in the year was 24:10, but there are plenty of better riders on better bikes, some did it in 19:something, so I've got a way to go!
As for comparing yourself to other riders in road racing conditions, see if your club does an evening chaingang for training during the racing season. Then ride with them, and see how long you can hold on before you get dropped. Later, you'll hang in all the way, and later still, you'll be going strong enough to look around you and see how much pain everybody else is in, so you'll know you are doing alright.
Have fun!