Polls in the United States open today in all 50 states and across 5 time zones. Timings vary by state, but polls open beginning at 6AM ET and close between 6PM and Midnight ET.
Of the 538 total electoral votes, half will have closed their polls by 8PM ET. 106 of those votes are in states that are considered competitive. 90% of those competitive states finish voting by 9PM.
If Hilary Clinton takes all the states where she is the expected winner plus the six competitive states that will have finished voting by 9PM, she could clinch the elections by then. The six states are Arizona, Florida, Georgia, New Hampshire, North Carolina and Ohio.
If Donald Trump sweeps every one of the 9 competitive states (including Nevada, Michigan, and Iowa), he could secure the 270 needed votes by 10PM.
As soon as polls close there will be a projection for that state, based on opinion polls carried out during the day, but the projections are sometimes in error. In recent elections, the major networks have refrained from calling the elections until after 11PM.