When you stop or terminate an EC2 instance, Amazon sends a soft shutdown request to the operating system to let it wrap up in a clean, safe manner. If the system does not indicate it is powering down within a short time (minutes) then Amazon effectively pulls the power plug forcing a hard shutdown.
I am not aware of any commitment from Amazon about how long this soft shutdown grace period is, so I would recommend you not assume or rely on having a specific minimum. Even if Amazon gives you 10 minutes today for one instance, they could easily reduce this to 3 minutes tomorrow when, say, they have a large demand for new instances.
If you need to do important wrap up before an instance shuts down, then send the instance a signal (web request or ssh command), wait for it to complete its task, then initiate the EC2 shutdown.
If you are using, say, spot instances where the instance can be shut down at any point by Amazon, then save your work frequently so that not much of it will be lost if the instance gets terminated suddenly.