My understanding is that ICAO says 2 mins. But Chinese ATC get awfully jumpy if you don't start down straight away. Generally they seem happy with anything over 1,000 fpm, but frequently they demand 2-2,500.
Look ahead in your flight plan and there's a few hints as to where they want you to have achieved that level by.
- FIR boundaries.
- Frequency changes.
- SID / STAR requirements. Many of the Chinese STARs say "At or above FL167" (for example). The ATCOs don't understand this level of flexibility. They only understand "AT FL167"
Get VNAV to level you off at that point and generally it works out nicely.