mussie
First the general answers:
1. PDI will expire after 10 days of no DEVELOPMENT ACTIVITY. This is anything that would show up in an update set. Updating data records does not reset the timer, updating a Script Include or Business Rule would. That said, there is a button on the Manage -> Instance page of the Developer Portal that will extend you to more 10 days whenever you click it, development activity or no.
2. Yes, an email is sent 2 days before the instance expires. This is subject to all the issues of email and might get caught in a spam filter or missed or something. The main thing I communicate over and over is to BACKUP IMPORTANT WORK FROM YOUR PDI. If it is in a scoped application, save it to a Git repo somewhere. Here is a video about doing just that. If it is not, save local copies of your update sets. Put exactly as much effort into this as the work is important to you. If your PDI is disposable, don't worry about it. If you've created an application you want to keep, have a backup. You could go into the hospital for 10 days anytime and then you'll lose the instance and all the work. On some regular cadence, take a backup. Last thing Friday is good, as is every time you stop developing on the app, etc. Whatever works for you, do that. Just make it regular.
In your specific case, I can't tell what instance you are talking about. You have two accounts, one with your SN email that appears to have never had an instance assigned to it. The one with your Gmail account has had two instances, the last one was assigned in December 2015 and reclaimed in January 2016. If you've had one more recently than that, I am not seeing it in my records or else it is in an account that isn't either of those two.
