Suggestion A  
30 minutes four times a week, i.e.: 20 minutes cardio, 10 minutes weights (1 muscle group, e.g. legs)

Suggestion B  
30 minutes three times a week
Mon:  20 minutes cardio + 10 minutes stretching;
Tues:  20 minutes weights (2 muscle groups, e.g. back and abdominals) + 10 minutes of cardio.
Wed:  20 minutes cardio + 10 minutes of
Weights (two muscle groups, e.g. triceps or chest, biceps or shoulders)

Suggestion C  
20 minutes 5 days a week.
Week 1:  all cardio
Week 2:  weights
Week 3:  Cardio on Mon/Wed/Fri
Week 4:  Weights on Tues/Thurs
Repeat the entire cycle when you get to month 2.

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