Usually at arms length. Put your arms up horizontally with fist closed, and you should barely be able to skim the ones next to you or in front. After a while you know the correct distance well enough that you don’t really need to use arms. Afterwards, the officer issues “adjust left” (or right), where everyone turns their heads and make sure they’re lined up perfectly.

The only exceptions were some special drill formations which were almost shoulder to shoulder (row), or “sausage in bun” (column)