We don't commit to providing any support for open-source users, and even for most commercial users, our formal commitments defined in the contract are usually quite limited. However, we don't normally take any notice of that: we do our best to help all our users, whatever the contractual relationship. The only time we admit defeat is (a) if you are using an old release of the software and can't reproduce the problem on a newer release, or (b) if the problem involves interactions with other products that we don't have access to.
If you find a bug we will usually fix it on the current major release branch; in a few cases if it's serious enough we might also fix it on the previous major release branch. Open-source users will immediately have access to the patch, if you are prepared to rebuild the product from source, but most users wait for the next maintenance release. Maintenance releases are typically issued every couple of months, but it depends on the number and severity of bugs that need to be cleared. To use a maintenance release of one of the commercial products, you will need a license key that has not expired; we normally issue license keys that give access to maintenance releases for one year from the date of purchase.
The open-source version of Saxon is hosted on SourceForge at http://saxon.sf.net/