+1 on the comments about full-body exercises. In particular if you incorporate balance work within the sessions, results improve a great deal.
in terms of frequency? IMO you should look more to what else you do that day - one day hard / one day easy seems an easy rule of thumb.
You may want to think about what your goals are - general fitness, tri training, crits, weight loss, etc. I had better luck subjectively when incorporating workouts into an overall goal. There are tons of good plans that make this easy.
One recommendation - stay away from crunches or sit-ups! They are the worst thing you can do for your spine, especially when you ride a lot. Planks or hanging knee raises are MUCH more effective and don't put strain on your spinal column.
Last edited by JCNeumann; 05-26-14 at 05:32 PM.