i lift about 3 days a week (and ride 4 days usually). i find it keeps me balanced and much stronger than riding alone and keeps the sunken chest pro cyclist look at bay.
usually i do two days of upper body and one of legs each week, mixing up my routine all the time though.
for legs i mostly do some leg presses and squats and don't bother with extensions and curls and such.
for upper body i mix it up a lot. i don't lift super heavy (usually 3-4 sets of an exercise and anywhere from 8 to 25 reps depending on the exercise) my weight workouts take about 45 minutes.
the hard thing is balancing overall training volume and recovery. you've got to be able to keep an objective view on how you feel and learn to listen to your body.