Not an extension, but there is a way through chrome's developer tools that you can preview the length of a bundle before you decide to buy it, I pretty sure this applies to Microsoft Edge and Firefox.
On your PC click F12, for Mac's use Command + Option + i
In developer tools click Network, if you don't see a list of messages at the bottom you will have to refresh your page (F15 on PC, Command + R on Mac)
In the search box or filter type "message" to find "messages?limit="
In the Response sub-tab you should look through the messages you have sent between the model and yourself then in media sent look for duration, it measures in seconds. For example, "duration": 215 is 3 minutes and 35 seconds.
If you get confused dm me and I can help, has saved me so much money from bad bundles.