I think the simple answer would be:
"When things are not coming into focus 100%".
You may never need your eyeglasses for stargazing, when distances at infinity are involved.
You need to experiment & educate yourself around your prescription.
Some binoculars are capable of focusing down to around 5' or so.
You may need your eyeglasses when focusing up-close.
Orion12x63MiniGiant+Spectrum I 20x65+Orion15x70LittleGiant II
Stan~Rocky Mountain High Colorado