The effect of depo on a cycke is quite variable.
Additionally, the effect is not cumulative. That means it will not take longer for them to resume if more shots were used. It is not uncommon for cycles to take from 4-6 months to return after even a single shot. Also, some may actually resume as soon as 6-8 weeks. If no cycle for more than 12 months after the last shot, definately needs eval.