It appears to be 3 days. See
Lost and damaged baggage | Baggage essentials | British Airways
Edited to add: (well hidden on their site)
The air carrier is liable for destruction, loss or damage to baggage up to 1,131 SDRs (approximately £1,000 or EUR1,230). In the case of checked baggage, it is liable even if not at fault, unless the baggage was defective. In the case of unchecked baggage, the carrier is liable only if at fault.
This is as required by EU Regulation (which implemented the Montreal Convention 1999) and applies to all EU carrier flights.
You may have to prove your actual loss.