You can DnD on the fly, but the goal is to have at least two 2h blocks booked before hand so you know there is protected time every day. You can always spend more focused time at your discretion.

This gives you at least 4h of deep work each day, which Cal Newport heavily supports in his book "Deep work" (http://forimpact.org/4-hours-deep-work-day/)

There is nothing worse than days when you are completely distracted and at the end of the day you ask yourself "What did I even do today?". This system prevents that from happening.