From a post on flight international (for DEC B777)
Requirements:
1. Hold current B777 type rating
2. Have logged at least 3000 total PIC hours and 1500 PIC hours on type
3. Last flight on type within 12 months
4. Valid Class 1 Medical Certificate
5. ICAO level 4 or above
6. No accidents or incidents reported
7. Age 55 or below
If you're an experienced FO then you'd need to meet the requirements of CCAR121 upgrading on large aircraft, plus have substantial experience on type.