It depends on your age, how long you have been riding, the wind that day and a dozen other factors. Serious TT guys (Cat 1) will do a 25mile/40km in 50-55minutes or faster. Most people would be happy with something under 1hr 10mins. Averaging anywhere over 21.8mph/35kph is good. Keep working at it, the times will drop quickly.
CHEERS.
Mark