"Stable and secure" is a subjective evaluation. "Unsupported", however, can be clearly defined in this context: the binary repository your system is tied to will no longer be maintained.
That means if the day after the support ends a new version of some software is released by its upstream developers, including (eg.) a critical bug fix because someone found a security vulnerability, then you will never see it via an automatic update, because there will be no more updates to anything in squeeze.
If you have not been keeping the system updated, this will obviously make no difference to you. However, if you do apply updates regularly, then you will now be out of the loop; there won't be any more.
So, if you consider such support integral to maintaining a "stable and secure" system, then the answer to "how long will this be okay?" is until you fall victim to an issue that you could have avoided via updates. That is the definition I would use, and it could mean 3 days, 3 months, or 300 years. No one is going to be able to predict or estimate that amount, as there is no data to analyse vis., how many such failures have occurred in relation to how many such systems exist, etc.
However, if you are not worried about keeping the software updated with security fixes, etc. then the answer is as long as you want. You can use squeeze for the rest of your life and still think of it as "stable and secure". Until something goes horribly wrong, lol.