What would you do if a client's sandbox refresh keeps failing or takes an unacceptably long time?

Refresh duration scales with what you are copying, so I look at scope before anything else:

1. Match sandbox type to purpose: Full sandboxes copy all data and are slow by definition. Most development and testing does not need production data volume — Partial Copy with a sampled template, or Developer Pro seeded deliberately, is usually the correct answer and refreshes far faster.
2. Use sandbox templates: For Partial Copy and Full sandboxes, templates control which objects are copied and how many records, which is the main lever on duration.
3. Reduce the source volume: If a Full sandbox is genuinely required, the archiving work that reduces production volume also reduces refresh time. The two problems share a solution.
4. Automate post-refresh: Sandbox post-copy scripts handle masking sensitive fields, deactivating integrations and outbound email, and reseeding test users. Failures here are often mistaken for refresh failures.
5. Check for genuine errors: Persistent failures rather than slowness usually trace to a specific object or a post-copy script and warrant a Salesforce support case with the failure detail rather than repeated retries.