proposed Format: 'Tu-Sa 08:00-15:00; Sa 08:00-12:00'.
see it here: http://www.geodatacheck.com/p/2
Use a semicolon to separate the different days or or time ranges. Use two letter english day names. Times are local times with leading zero
This is the format as used in openstreetmap.org for a node description with opening hours. Openstreetmap has very rigid semantics, so it makes sense to follow their format.