Dust 'on' the pc won't hurt it in the slightest. Just look ugly. Dust 'in' the pc is a different story. Variations in dust 'thickness' will affect different pc setups differently. I've seen plenty of stock HP and Dell pc's with dust so thick it's a blanket covering up the roaches who had tunnels running every which way. Didn't affect them really as there wasn't enough power draw through the mobo or other components to worry about excess heat. I've also seen high-end pc's with hard OC's pushing components to their limits that show marked temp increases with just a thin film of dust, even loosing stability they were dialed so tight. With an i5-7400 and 1050ti, the looks of the inside will be more a factor than possible performance losses until the cpu/gpu coolers get clogged.
When to dust? Realistically, when it's dirty. That'll be decided by your personal pc hygiene insistence, environment, pc setup (amount of filters, airflow) and temps. I know what my idle temps are and certain game temps should be, if they go up, time to clean for sure. You'll figure that out with time and experience with specific games and scenarios.