How long to hear back after 1st round?

There's no rule of thumb. Every firm and process is different with limitless variables that impact timeline and outcome (team's schedule, holidays, other candidates, travel commitments, etc.). "First round" doesn't even mean the same thing across shops (coffee chat versus single interview versus model test, etc.). Best of luck.

If its through a HH you should receive informal guidance fairly quickly (ifs it positive), usually in a day or two. "They enjoyed meeting you, standby for next steps" something to that effect is common to keep you engaged.

Formal guidance on actual next steps, etc, is highly variable.