In addition to what Lynn has shared, the answer depends on:

how good the DBA is at automating things

are you willing to put into place and pay for a third-party monitoring package.

Do you expect the DBA to analyze and fix code if deadlocks are occurring?

Do you expect to the DBA to performance tune queries

Frankly, if all you expect is the DBA to keep things up and running, once monitoring is in place and tasks are automated I'd say less than 2 hours a day because for the most part they should be checking a dashboard in the morning that has no issues and then answering SSC questions the rest of the day.