In this post (https://forums.plugintolinux.ca/index.php/topic,561.msg3495.html#msg3495) in a topic on the new Linux Mint 19, Buster asked how much space the Timeshift backups use. I have given an answer that it depended on how many snapshots you have which depends on the schedule you set up for backups and how many you keep. Which it obviously does still but here's my example to give you a ballpark.
I set Timeshift to backup daily and weekly and to keep 5 daily backups and 3 weekly backups. Since I started using it, I now have all of those plus one on-demand backup which the screenshot shows. Backups are saved in /home/timeshift the way I have it setup and the entire directory takes up 13.9 GB. I'm only backing up the system, not /home, which is how it is setup by default and the size of that is 8.4 GB.
So the answer is, not a lot considering you're backing up the entire root (minus home), not even twice the size for 9 backups and that's without using compression. Note that this is obviously using one complete backup for the first snapshot and then incremental changes for each of the others. Only the files that are new or changed are backed up in each incremental backup.