In some circumstances long-term sickness absence may be defined as 2 weeks or more, but in the NICE guidelines long-term sickness absence is defined as an absence lasting 4 weeks or more. Short-term sickness absence in the guideline is defined as an absence lasting up to (but less than) 4 weeks.
NICE goes on to say that in the guideline recurring short-term sickness absence is defined as more than one episode of short-term sickness absence, each lasting less than 4 weeks, and that recurring long-term sickness absence is defined as more than one episode of long-term sickness absence, with each episode lasting more than 4 weeks.
Cite this: Dr Rob Hicks. Quiz: Fit Notes, Long-term Sickness, and Work Capability - Medscape - Jan 24, 2020.