There is no legal timeframe that says when you must renew DBS checks.
However:
The DBS states that renewing checks every three years is considered best practice.
As an organisation:
You decide how often to renew DBS checks for your existing staff.
You should base this on:
Your safeguarding responsibilities
Your risk appetite
The roles and activities your staff perform
Sector-Specific Renewal Requirements
If you are regulated by a specific body, you should also check their rules.
For example, you may be regulated by:
Ofsted
CQC
These regulators may:
Set specific renewal timeframes for DBS checks.
You should:
Review their guidance
Align your own renewal policy with any sector-specific requirements that apply to you.