I have a velodrome and a "circular training track" (velodrome without proper banks) close to home - the second is on my way to/from work on the bike so is perfect.

What do you want to get out of your training? Or (as said above) are you looking at mixing it up.

To stress your aerobic capacity and fitness, VO2Max intervals are very good - requiring 3-6 minutes or so at the highest intensity you can. I have a power meter which makes it easier, but equates to sitting on approximate 40km/h while on the hoods for me if I didn't have one. If you are doing them right you should need to bail on the last one or unable to start after 5-6.

Shorter will affect your neuromuscular and anaerobic systems more.