I was in the process of checking how much Spotify is writing to my SSD (following this article). What I discovered was about Toggl Desktop: it wrote more then 500GB to my SSD in just 12 days. For comparison, during the same period of time, Spotify wrote around 200GB (still a lot), Dropbox wrote about 20GB and all my browsers combined wrote less than 20GB.
The overall rate at which toggl writes to my disk comes out to be something around 45GB/day or close to 2GB per hour. Of course, my computer is not on 24 hours, so the actual rate is at least twice that.
Here are some details on my system, in case it's important:
Hardware Overview:
Model Name: MacBook Pro
Model Identifier: MacBookPro11,1
Processor Name: Intel Core i7
Processor Speed: 2.8 GHz
Number of Processors: 1
Total Number of Cores: 2
L2 Cache (per Core): 256 KB
L3 Cache: 4 MB
Memory: 16 GB
System Software Overview:
System Version: OS X 10.11.6 (15G1108)
Kernel Version: Darwin 15.6.0
Boot Mode: Normal
Secure Virtual Memory: Enabled
System Integrity Protection: Enabled
Time since boot: 12 days 50 minutes
Toggl Desktop Details:
Version: 7.3.337
Record Timeline: Enabled
Autotracker: Disabled
I'm also attaching a screenshot of the preferences pane, to save me from entering all that stuff manually.

I can confirm the desktop client to write around 2 gb per hour while I had it only on standby during the weekend:

I would love to see this fixed.
The data is written when timeline is enabled. Timeline tracks actions every second and saves data to local database. This is why the amount of data written is big. Disabling timeline will lower the written data amount greatly.
@IndrekV even taking this into account, it seems like an extraordinary amount of data. For comparison: iStat Menu keeps track of all my system load (CPU, Memory, etc.) and samples every second as well, but only writes about 4MB per hour to disk.
Even Spotify (which is getting slammed right now for writing too much stuff to disk) is writing at half the pace that Toggl Desktop is, and that is considered too much for an app that caches music.
If this about of disk writes is necessary for having Timeline working, then its usefulness is overshadowed by the drawbacks. However, others may disagree. At the very least the users should be warned that large amount of disk writes will happen if they turn "Record Timeline" on.
Thank you for your feedback. We will re-evaluate how we handle the timeline data and see how we can improve this.
When I asked the support two weeks ago, Kadri suggested clearing the app cache which helped for some time, but now the Toggl is writing like mad again. (W10 64bit)

@IndrekV anything new?

Sorry nothing yet. We haven't managed to dive into this issue. We will do our best to start working on it as soon as possible.
Another person here that can confirm that it's happening. I'm going to try disabling the "record timeline" feature, but feel it's worth mentioning that it didn't actually sync any of my unspecified time. From the description in the docs, it sounded like this was supposed to keep track of my time when I wasn't actively recording a task, but there's no data when I'm logged into the website.
I actually wouldn't have minded the 220gb it apparently wrote since I last rebooted much if it actually recorded that data and I could analyze my "slacking off" time better.
I've added timeline data cleanup. It seems that old timeline data was what was causing a lot of read and write actions. With this change unusable and synced timeline data is cleared on app start.
First version with the fix is https://github.com/toggl/toggldesktop/releases/download/v7.4.14/TogglDesktop-7_4_14.dmg
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Thank you for your feedback. We will re-evaluate how we handle the timeline data and see how we can improve this.