Germination – Schoolboy Question? (No offence to schoolboys)

Depends on the temperature of its surroundings.

At 5C onions take 31 days to germinate
10C = 13 days
15C = 7 days
20C = 5 days
25C = 4 days (but less seeds germinate)
30C = 4 days (even less germination)
35C = 13 days (only 73% germinate).

Don't ask me where I found this information, its a chart I printed off years ago - it'll be on goggle somewhere.

From here maybe? https://tomclothier.hort.net/page11.html