Sample code for this can be found here.

There is no official doc, but this is a good source. Basically, even if the browser is opened and the computer is awake, the maximum runtime is 12 hours.

• The 'maximum lifetime' of a running notebook is 12 hours (browser open)
• An 'Idle' notebook (browser closed) instance cuts-off after 90 minutes
• You can have a maximum of 2 notebooks running concurrently
• If you close the notebook window and open it while the instance is still running, the cell outputs and variables will still persist. However if the notebook instance has been recycled, your cell outputs and variables will no longer be available.

Given this, the best approach seems to use Keeping you Awake and leave the browser open, run the script in a mounted Google Drive and save the model each epoch. Then, even if the sessions ends and the local data is lost, the model still should have been saved to Google Drive.