Unless its a vocational (LGV/PCV) licence then there is no requirement as such, only suggestions.  You're the company, you could decide when they are done, from none to any other time period.  I favour at the same time as driving licence checks, typically 6 monthly (low risk) or annually depending on the role, and of course after an incident/ collision.  If there is a failure then remove from driving and to be cleared by optician before resuming.
Three main ways - typical No plate in car park or yard at about 20.5 metres, use a Snelling chart or with Vutest.com -I think they now charge but so easy to do. Keep a record they are done, if glasses required (check driving licence for code 01) then make sure the check is done accordingly.