@Sara6 - from everything I've read on the board and in the book, IF will not slow your metabolism. What will slow your metabolism is chronic caloric restriction.
IF is/seems to be a very personal decision and how you do it needs to work for you. For me, I IF once per week, from Dinner on Friday to Dinner on Saturday. Others on the board have written about skipping a meal every day.
Look around and read the others posts, experiment and do what works for you!