Both of the summons don't disappear. More often you can meet Frost Spiders because it has more HP, all of Frost Servants are usually killed with one or two hits (especially when players has higher level). Anyway, I met them both at once after Zushuka was killed.
I have no idea how long it may stay on the respawn. Fire_Draggon said 3 hours like other raid's creatures, but ... Orc Marauders from Zao raid can disappear after 20 minutes. I wouldn't be so sure what is the exact time in case of Frost Spiders and Frost Servants :P All I can say is that I met Frost Spiders about 1 hour later, after Zushuka's death.
EDIT: Okay, I checked it on my own - now I can say that both Frost Spiders and Frost Servants stay till ss.
The moment when you meet Zushuka as first, you can notice that she is standing alone with no summons. You can even easily miss her because there are no additional monsters and she is very similar to the Ice Witches, so: when you can see summons it means that Zushuka has appeared and she already got some hits or dealt some hits. A few years ago I used to check the whole cave because it was probable, that Zushuka was following the bot and when botter died, she was lured away but still alive. Nowadays you rather won't meet any botter in this cave, so it is a sign that Zushuka has rather died. Anyway, you can still check the cave to be sure in 100%.