When does the Reg CF clock start and end for the raise limits each year? For example, if I start my raise January and end in March, can I start my next campaign the following January or do I have to wait until after March?

It is a rolling 12-month period.  So you can start a new one as long as you are under the threshold and you have the needed financial statements.  Usually you will need to prepare new financial statements every April 30 (for calendar-year companies)