20-40 mins isn’t long at all. Some broodies go off them for a couples hours
and you can candle any time 
although that said the lockdown thing mostly applies to eggs being incubated in an incubator because it can mess with the temp and humidity opening it too much but with a broody that’s not really a worry and even in the incubator, some people candle every day even hatch days and they’re fine.
