Handling a supplier who is usually good but has become unreliable

Our packaging supplier used to deliver on time with almost no chasing. Recently three orders arrived late and the last shipment had missing items. I don't want to abandon a longstanding relationship over a rough month, but our customers don't care which company caused the delay. I'm planning a direct conversation, clear lead-time confirmation and a backup supplier for critical stock. How long would you give a previously reliable vendor to recover before shifting meaningful volume elsewhere?