How long until eggs hatch?

[Sachi1]

I put eggs under a broody on the 4th of February. 4 hatched,and 2 survived on the 28th or 29th..She still has 5 eggs under her as of today, March 4th. How do I know when they aren't viable and won't hatch? How much longer should I leave them under her? I have felt the weight of them and notice some feel heavier than others. I've heard about candling eggs, should I do that and how?? Sorry about all the questions, this is our first hatch.