| No. Queens | Time (ms) |
|------------+-----------|
| 1k | 1 |
| 5k | 1 |
| 10k | 2 |
| 50k | 8 |
| 100k | 22 |
| 500k | 42 |
| 1m | 122 |
| 5m | 1492 |
| 10m | 1904 |
| 50m | 9923 |
| 100m | 20649 |
| 200m | >300000 |
| 300m | 86049 |
| 400m | 148276 |
| 500m | 203613 |
| 1b | 247545 |
| 1.5b | 550431 |
| 2b | Failed |
| 10b | Failed |
I ran your program with powers of ten up to 1 billion queens (./nq 1000000000) and then attempted 10 billion (./nq 10000000000), but the process simply hung after 30 seconds. After that I went back and tested some intermediate values, up to 2 billion, which failed. Additionally, 200m appears to get stuck forever. A linear regression on values on values from 100m to 1.5b reveals an r of 0.9656, which is fairly high.