Welcome to the Apple Community.
It needs doing all at once.
The first back up to iCloud may take some considerable time. Just how long isn't really possible to say without knowing how much is in your back up and your connection speed.
What many people don't realise is that upload speeds are often significantly less than download speeds, so for example a user may have a download speed of 40 Mbps, but only 1 Mbps for uploading. As a rough guide it will take around 2 ¼ hours to upload 1 GB of data at 1 Mbps.
Being able to back up to the cloud can be very useful, especially if you don't have access to a computer or have infrequent access to one, however unless you specifically need to use iCloud for back up, you will find backing up to iTunes significantly more convenient and possibly more reliable.