It depends on who you ask, but you're looking at a consensus minimum engagement distance (MED) of around 15 metres and recommended engagement distance (RED) of around 20 metres, and in semi-auto mode only.
A variety of posts in this forum's thread, using 0.20g BB ammunition at 450fps, mention variously
- 50 feet (15 metres) (Utah)
- 20 metres (Brazil)
- 10 metres (Indonesia)
- 40 feet (12 metres) (Texas)
- 20 metres (Norway)
Websites for airsoft associations in other countries describe similar limits, MED 15-20 metres and RED 20-25 metres. You may be best advised to start at the high end of the range and work down as you develop comfort with being hit and judgement of appropriately safe distances.