At the shows I've done, they give me just enough time to ride around the arena one time in each direction. Depending on your horse's personality, decide in advance if you're going to let him take a look around in his better or worse direction. For me, it was worthwhile to get my horse around first on his non-spooky side and then do the spooky side once he had some confidence and I had his attention.
Also, don't panic if they ring the bell to start when you're at the oppositie end. 45 seconds is more than long enough to trot to the entrance and do a nice little balancing circle before starting.
Above all...have fun!!
You just have to see your distance...you don't have to like it.