They specify LTS (Long Terms Stable) on certain versions so that means that you can use them for a long time.  How long?  In the OpenERP Release Policy FAQ (https://doc.odoo.com/v6.1/contribute/14_release_cycle.html/) it says:

"The OpenERP Publisher's Warranty contract covers the 3 last LTS version in parallel. This means that when a new LTS version is released, an older version reaches its end-of-life, and is not supported anymore. As an example, 6.0 LTS will be supported along with 7.0 LTS and 8.0 LTS, but will reach end-of-life when 9.0 LTS is released."

So until v8 is released v5 is still supported and that was released in April 2009.  When v8 is released then v6 will still be supported and it was released in January 2011.  So you have support on those from about 5.25 years to (currently) 3.5 years and who knows when v9 will be released?  I havve to guess not until at least 2015 so that would give 4years of support and I bet it is longer and approaches 5 years (just a guess).  Check out this link and scroll down to "Release history" to see some info in a pretty handy format:

http://en.wikipedia.org/wiki/Odoo

That being said, will Odoo start releasing versions closer together?  Possibly, but even if they get down to 1 per year then you'd have 3 years of support on each version.

If you are still worried about having to upgrade to an Enterprise Contract after 3 to 5 years then check out companies who might do an upgrade for a set fee without the contract.  Check out this link which I got recently:

https://madmimi.com/p/aeb715?fe=1&pact=24021328602

And please let me know about your success with non-contract upgrade.  I have the same question and I woudl especially like to hear about approximately how much they would charge for such a service.