Just to add on: an online unfreeze is instant, and all of the major agencies also let you unfreeze for a specified date range (even down to 1 day, I think -- although I usually do 2 or 3 days to cover a second pull if something goes wrong with whatever I'm applying for).

As a side note, if you use the online instant unfreeze, note that at least one of them (I can't remember which) handles timezones extremely poorly, and if the "start" date of your unfreeze has passed in their home timezone, they'll reject the unfreeze with an extremely vague error. You can resubmit using "tomorrow's" date and it will work fine (and be unfrozen immediately).