Currently 3 years, but the Management of Health & Safety Regulations require your employer to keep you competent throughout the 3 years.
The First Aid Regulations are changing soon: the 3 year validity will stay but a certificate will only be current if you undertake a certificated 3 hour course yearly. The normal requalification course must still be undertaken at the end of 3 years, like now.
All HSE approved organisations can run them, so with a St John certificate you could do your 3 hour refresher through another organisation, like
http://www.firstaidtraining.org.uk
Hope this helps