I would think you should consider the legal position,
There is a substantial and complex amount of EU and UK legislation which has an impact upon the retention of personnel and other related records. Examples of legislation dealing with particular categories of records are provided in the boxes below. Other important statutes, statutory instruments and EU Directives include the following:
Acts
Limitation Act 1980
Data Protection Act 1998
Freedom of Information Act 2000
The Regulation of Investigatory Powers Act 2000
Statutory instruments
Privacy and Electronic Communications (EC Directive) Regulations 2003 (SI 2003/2426)
The Regulation of Investigatory Powers (Acquisition and Disclosure of Communications Data: Code of Practice) Order 2007 (SI 2007/2197)
The Data Retention (EC Directive) Regulations 2007 (SI 2007/2199)
Directives
Data Protection Directive 95/46/EC
Privacy and electronic communications Directive 2002/58/EC
If you are not subject to any of this legislation then select a reasonable period that you think acceptable.
Or just keep them for 5 years as P has said.