The seller chooses the days and the times. You see them before you bid, which is the point — collection is a commitment you make in advance rather than a problem you solve after winning.
Where they are
On the listing's delivery information before bidding, and on the order afterwards as Hours: {hours} and Open {hours}.
A store sets this up as Pickup days — either open every day, or specific days with an opening and a closing time. How to offer pickup
Check them before you bid
This is the whole message of this page. A pickup item you cannot reach during its hours is an item you should not bid on, and finding that out afterwards is the commonest way a collection goes wrong.
You accept the collect-by terms before bidding rather than after winning, and the hours are part of what you are accepting.
How the hours interact with the clock
The collect-by time starts when the store marks the order ready, not when you pay. And days the store or the point is closed do not count toward it.
So a store closed on Sundays, with a 72-hour collect-by on an order ready at 5pm Friday, is not expecting you on Sunday. The clock pauses for their closed days.
That is more generous than it first looks, and it is why "I only have weekends" is sometimes workable with a seller who is open on Saturdays.
If their hours do not work for you
Ask before bidding, not after. Sellers do accommodate — "I could do 7pm on Thursday if you tell me the day before" is a common answer, because they would rather hand it over than deal with a no-show.
What they cannot do is be there at a time they are not there.
If you are going to be late
Message the store before the collect-by time runs out. They can extend it — a store can give more time, never less — or offer you a courier price that turns the order into a delivery. I missed the collect-by time
Holidays
Stores set closed days and holidays, and those do not count toward your clock either. A seller who has filled that in properly is one whose stated hours you can trust; a seller who has not may be shut for a festival with the clock still running, which is worth a message rather than a wasted trip.
The practical habit
Before you bid on a pickup item: read the hours, decide which specific day you would go, and check that day is open. Thirty seconds.
More: where exactly do I collect from, how to collect an order, how pickup works.