Have it sooner than feels comfortable. The longer a conversation about performance, attitude or behaviour is put off, the bigger it grows in everyone's head and the harder it becomes. Avoiding it also quietly signals to the rest of the team that the behaviour is fine, which makes your job harder later.
Keep it short, factual and kind: name the specific issue, give a clear example, explain the impact, and agree what changes from here. Do it privately, and follow up in writing if it matters. If it's sensitive or you think it could escalate, plan the conversation with someone experienced first so it lands well and protects you.
Every situation has its own details, and those details change the right answer. For a read on yours, take the free Employee Situation Check or book a call.
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How to handle this, in a couple of minutes
Samantha talks through the practical steps.
General guidance to help you decide your next step, not legal advice. Employment law changes, so always check the current position for your own situation.