Maximum memory setting on database instance

In reference to the database server properties -> Memory -> Maximum server memory (in MB).

The context is I have two SQL Server instances (one for SharePoint and for data), as well as SharePoint, SSRS, and SSAS running all on the same virtual machine.  There is 24GB of memory on it and 4 processors.  While the goal is to start splitting stuff off on different servers eventually, my question right now is what recommendations are there for what the maximum memory should be on the two SQL Server instances.

Thanks.