Every week the events changes, its in Hot Topics.

The same type of event might take around 4 weeks to 8 weeks to be repeated.

Like for example Refinement Rebate or Sakura event reappear like once a month or 2 months?

There is no telling what ninjas will reappear in ninja related event. But every week different ninjas appear in different redeemable event.

You just have to stack and keep stacking coupons until a good ninja event pops in, since not all ninja events are cheap even if they appear.

But there will come one, like this Sakura event with Jonin Minato. I only spend 3.8k coupons to complete my remaining frags to get up to 80.

Something all active F2P can achieve. Keep stacking coupons, read the news sections every week.