Not long, but it's not something you should be doing if users are accessing the DB.

Stop the service.

Start SQL in single user mode (takes probably as long as it takes to start the service)

Connect to SQL and restore the master database. Note, SQL's in single user mode. If someone else gets a connection before you, then that person is the single user. ie, make sure you do this when no one else will be trying to connect to the DB

Stop SQL (Ctrl-C in the cmd window that's running sql)

Restart the service.