What constitutes a late check-in for a hotel and should I inform them about my late arrival?
For a particular hotel I booked Expedia's Check-in policies section states the following:
Check-in time starts at 4 PM
If a late check-in is planned, contact this property directly for their late check-in policy.
My question is... what constitutes a late check-in? My flight is scheduled to arrive at the airport at 8:34pm.
Thats down to the individual hotel - you need to ask the hotel that, as Expedia suggests. An 8.34pm scheduled arrival at the airport means you probably wont be at the hotel until 9.30pm, probably later - so yes, I'd get in touch with the hotel.
I don't think there's any standard - you'll have to check with the hotel directly. Major chain hotels have 24 hour desks and probably won't need anything special; bed-and-breakfasts may have their staff go home after dinner and you have to do something special to get a key if you arrive later.
A room is usually guaranteed until a certain time. After that time, you are considered a no-show and it may be rented to someone else. For this reason, most hotels have a check-in end time. Otherwise, it will state Your room is guaranteed for late arrival, usually in cases where they have already taken the payment, or at least enough for the first night.
You have not specified the end of check-in time, which sometimes is not even provided! It depends on the particular hotel but something between 10 PM and midnight is common. Any time you expect that your arrival will occur during or after those hours, simply call the hotel and let them know that you are coming and an approximate time. Do not forget to factor time to disembark, collect luggage, customs, immigration, waiting for transport and getting to there.
There is another case which is usually specified clearly and that is when check-in is impossible after a certain time. The only times I saw this was for a few small properties in Taiwan where the front desk is not open after certain hours since the attendant goes home. Early check-out also involved leaving the keys in an agreed-upon location.