Does Ubuntu have a fixed package update cycle like Microsofts patch Tuesday? Or are packages just updated and released ad hoc. I've checked around and can't find an answer which makes me think they are released as and when they are needed.
They are released as and when they are needed.
Users will see security updates immediately. But otherwise, the Software Updater will (by default) only notify users once a week.
There is additionally some experimental support for phasing updates out to a higher and higher percentage of users over time. This isn't widely used yet, to my knowledge. But it's built into the infrastructure.