I don't know if there is a specific event that favors a time to buy the jonin medal. Maybe the bimonthly Jonin Renewal like someone suggested.

I would def wait for a solid recharge event where you get lots of coupons for -only- recharging. Watch out for the scam that requires you to recharge + spend ingots to participate.

As far as spending ingots goes, there is never enough patience. I believe the best return on spending is to renew the monthly coupons if you truly do not wish to invest further.