I rode 25-32 hours/month, 8 1/2 hours being my biggest week, for a total of 3500 miles so far this year. I started the season as a Cat 5, and now I am a Cat 3, doing mostly road races. I agree with merlin - if you can do 3 hour long rides during the week and a longer ride on Sat. or Sunday you can be competitive in the Cat 5 or 4s. I think the key to training on limited time is to make sure your rides are focused and are either really hard, or really easy.