We've just hit today's cap, and the submission form has turned itself off for the rest of the day. It'll reopen in about six hours, at midnight Eastern US time.
Because of various factors, including the several (off-peak) hours when the form was mistakenly offline this morning, it's hard to usefully say how long today's open period lasted. But I think I can say it took somewhere on the order of ten hours to reach the limit.
If I had to guess, I'd guess that tomorrow we'll be open 'til about noon Eastern US time. But that's an extrapolation based on way too little data—and the fact of my making that prediction probably changes the outcome, because people will probably see this note and aim to submit earlier in the day than they otherwise might.
As I mentioned earlier, the cap will probably continue to prevent some people from submitting on their first-choice day for the next week or so. I suspect by the end of next week you'll be able to submit at just about any time of day or night, but we'll see.
Also as I mentioned earlier, I've now sent autoresponses for all submissions we've received. If you submitted today and did not receive an autoresponse, please query immediately.