Usage over the last 6 months isn't stored.
The options are usage over the last month via a Self-Help Service Status Health Check or a request for a manual check for the last three months' usage carried out by the TalkTalk Community Team OCE.
Try carrying out a Service Status Health Check.
You can ask here for your Broadband usage over the last three months. An OCE will be able to do that for you. Please ensure your Community Profile Personal Information (Click here) includes your name, address, current TalkTalk landline 'phone number and an alternate number (mobile recommended). Add, then scroll down and save changes. The TalkTalk Community Team OCE will then be able to link to your TalkTalk service account.
Here's the info requested .
12/2018 = Download 253710 MB Upload = 37018 MB
11/2018 = Download 231151 MB Upload = 31090 MB
10/2018 = Download 122254 MB Upload =19980 MB