During top up events with rebates. Such as feast wheel or slot machines. Best value for your money tbh. Spending events are good as well but it depends how much you're planning to spend weekly/monthly.
Well, to be honest this would be a good moment to recharge, since the current event has good rebates. ;P I'm not saying you should or should not recharge, that's your choice though.
Wait for an event that rewards you for recharging and doesn't require you to spend. So for example: an event that rewards you when you recharge 1,000 ingots, but then you can spend those 1,000 ingots on whatever you want.