How late will I have to stay up to know the next president?

[Erc]

Early returns from Indiana and Kentucky between 6 and 7 PM EST will indicate whether Trump is under-performing in Republican counties. Early returns from Florida between 7 and 8 PM EST will also be telling. I imagine states like Pennsylvania and Virginia where she is leading will not be called immediately just in case there is some hidden Trump vote, but we should know before 9 PM EST how the two candidates are performing in relation to polling and it will be clear who is going to win unless the race has drastically toghtened.

Problem with VA is that NoVa, especially Fairfax, is really slow to count. It is likely that Trump will have a lead in the state early on in the night.

I'm curious why that is.  It's densely populated and near a global data hub.

Here in Arlington, at least, we have a lot of very large precincts, and the state specifically cut off money to modernize the voting system several years ago (for a while, this meant there was a confusing mix of electronic voting machines and paper scantron-type ballots; we've since switched back to scantron entirely).

We also have a very large itinerant population, with resulting large swings in turnout that we aren't always well-prepared for; I believe we ran out of ballots on March 1.

As a result, expect long lines that mean voting won't be finished until well after 7PM, plus the by-hand verification of the results typically takes an hour or two.

[Erc]

In terms of mathematical certitude (Clinton has 270 EV called for her):

Somewhere between 9 and 11, depending on when NC & GA get called for her.  NC is typically pretty slow, so 9PM seems too early.

Obama 2012 - West Coast + GA = 270 EV, so 10 PM is possible, but that would require every single state in there to be called by then; it's likely there'll be a fluke somewhere in there (Iowa? ME-2?) which would prevent a call when Nevada & Iowa come in.

I'd put my money on roughly the 10:30 PM range, though 11PM is also quite possible.

As for when any reasonable viewer would be certain: possibly as early as 7:30, if Ohio is called instantly...but it should definitely be obvious at 8PM when Florida, PA, etc. are called instantly.

[Erc]

Based on our discussion in the demographics thread, I expect the exit polls are likely to be too favorable to Clinton, as they tend to oversample college-educated voters (a split that actually matters this year). 

That, combined with Clinton's general landslide, means that I expect the media are going to be pretty aggressive in calling states, at least in the first hour.  Even if they have some incentive to string the election out / be responsible with their calls, they're going to call Ohio at 7:30 when their exit polls show Clinton up by 15.