The minimum bend radius of a fibre optic cable is set by the manufacturer and printed on the datasheet, so that figure always governs. A widely used rule of thumb is about ten times the cable outer diameter for a cable that is installed and at rest, and about twenty times the outer diameter while the cable is under pulling tension.
Bending a fibre tighter than its limit raises signal loss, because light escapes at the bend. A severe bend can also crack the glass, which may not show up until some time after installation. Bend-insensitive fibres, such as the G.657 type used in many drop and indoor cables, tolerate tighter turns, but the cable and its protective sheath still have their own limit.
Keep routes free of kinks, avoid pulling cable round sharp corners, and use proper bend-radius guides at trays and cabinet entries.