Updates are released about every 4 weeks, or less as soon as we finish a new feature we have been working on. With the free version, you do not get the latest updates or minor updates aka daily builds. E.g. the latest engine version is 1.5.3, customers on the Free Plan have acess to 1.5.0. Also, the latest major updates like the upcoming version 1.6.0 is only available to Pro Customers immediatelly, and with about 1-2 months delay to the Free Plan customers.
You can see all previous engine updates in the changelog here.