But it's better to do them regularly.
I have just under 800GB to back up from this computer to an external HDD (USB3) with Duplicati doing the backup, indexing files etc. It's been going since around 1.30am this morning, and is about 40% finished. Most of that is images & related sidecar files. :-P
As of Monday morning it's about 85% done. I left the Dell behind to complete and took the Macbook to work instead: it's really odd using OSX again, and a bit clunky after using W10, plus everything runs ssllloooowwwwwwwlllllllllyyyyyyyyyyyy despite the SSD. Guess there's no substitute for raw computing power.
Backup completed around 1.11pm this afternoon - that's 2.5 days almost exactly for >700GB data.