There are no plans to set standards for judging visa applications as complex at this time.
Visa applications are considered to be complex when an Entry Clearance Officer determines that additional information is required in order for a decision to be made. This usually means that the applications will take longer to process than the standard timescales.
In such circumstances UKVI will write to the customer within the standard processing time and explain what will happen next
The published information on processing times for visa applications is published as part of the Migration Transparency data, available at https://www.gov.uk/government/collections/migration-transparency-data#uk-visas-and-immigration