[TAL]

Last Saturday, I installed a frame of capped brood with two swarm cells to replace the package queen that failed to impress her sisters and they killed her.... I was told by the donor they would hatch either 4/20 or the 4/21....that being true, how long should I wait to check the frame for evidence that an emergence happened already?

When you placed the frame of capped brood w/cells into your hive did your have have any eggs/larvae or capped brood of its own?
Also were there any eggs on the capped frame you placed into the hive?

 

[TAL]

I would be careful and check the frame that you added with the cells on it. If you see eggs at any point in the inspection close it up. She is there and doing her job.
I ask about larvae and eggs on the donated frame because if the cell was damaged or not accepted they may have built another on that frame in a different location. Pending it could mean another cell not due for anothe 10 or so days. Just something to think about.

A more direct answer would be today or tomorrow unless you can hold out for saturday.

 

[TAL]

If you have a spare frame of brood it would be better.