With a gigabit internet connection, it would take about a day to backup 8TB. 100mbit could do it in 222 hours. At 10mbit, about 90 days.
That's to say, there's more to it than upload speed, if Backlaze had servers in Europe I'd recommend it much more than I do.
I have several TB's of other data (mostly photos, videos, etc) that I'm fine waiting for weeks or months for if needed, but if you're in a hurry, Backblaze will sell you a hard drive that they restore your backup to and mail it to you.
Or I could just sign up for a cloud backup service, spend a month or so uploading my initial snapshot of data, and then backups are automatic and always offsite, replicated, and scrubbed. For $10/month.
Plus you only do it once. Differential backup would take down the size of the next backups to a few Gb, or even Mb.