How long should I wait

Hi Topcat. Not sure about the answer as I have never hatched chicks. It may be an idea though to post this question in Hen House forum which is more for this type of question. I am sure somebody more knowledgeable than me will be able to help soon.

She will usually decide after a few days to leave the rest of the eggs.  If they are ready to hatch you can usually hear chirping from the egg.  You could always candle the remaining eggs as they may not have developed. 

Put water and food close to the hen so she and the one chick can reach it.

It is a difficult time of the year for hatching so one is quite good.  Any photos of your new chick and Mum of course.