If I install OpenBSD 4.9, when will I have to upgrade to 5.0? When will the 4.9 release no longer be supported by the OBSD teams?
The security support for a given version in OpenBSD is 1 year.
From the FAQ:
You will also note that in the above example, the 4.6-stable branch came to an end with 4.8-release, and the 4.7-stable branch came to an end with 4.9-release -- old releases are typically supported up to two releases back.
Make sure to join critical OpenBSD mailing lists to be made aware of upgrades and fixes.