Do I have to fill it out all at once?
Fortunately no! You can log in and out of Apply, amending details each time you do so.
There is no time limit for completing your application once you’ve started on it, but make sure you are aware of the deadlines so you send it off to UCAS on time.
Remember that as soon as you’ve sent off your application to UCAS, you can no longer amend it.