How long does sp_blitz take to execute? I'm trying to run sp_blitz on SQL Server 2014 and it seems to be taking a long time. What is the normal length?

We only have around 30 databases. One is 300GB, but it's going on for 15 min.
Can be anywhere from 5 seconds to 15 minutes depending on your server, workloads, databases, etc. If you have thousands of databases, for example, you can check out the documentation and use switches like @CheckUserDatabaseObjects = 0 to go faster by skipping stuff.