Each campaign has a daily send count, and this determines the number of mailings that will be sent each day once you activate your campaign.
Once each day ('round midnight), Pebble will gather your active campaings and send a batch of mailings according to the campaign's daily send count. Mailings will be sent in order according to the row number of your list file.
Mailers go out everyday at 2:00am UTC (7pm Pacific, 8pm Mountain, 9pm Central, and 10pm Eastern)
If you'd like to send an entire campaign in one day, simply set the campaign's daily send count to a really large number.