What is the best time to apply for a PhD in physics?
For a physics PhD in the USA, experience after a four year bachelor's degree is not expected.  However, if you have a three year bachelor's degree or your degree comes from a country with many applicants like China or India, a master's degree may be very helpful.  You should make an application plan no later than the August 1 before you graduate.  This allows time to prepare for the September Physics GRE.  Applying the year following graduation should be a backup plan.