Effect of job change on perm application after I140 approved

I have my I140 approved last year in June 2015 with priority date at June 2014 from company A and I have an offer from a company B, which I am planning to accept. . My new employer told me that they will file my perm application . I had few questions :

  1. If somehow my new employer does not file my perm application and I change my job again to employer C Will employer C be able to apply my perm application and will I be able to retain my priority date ?

  2. Is there any time period in whch perm application has to be applied once last perm application was approved ?

  3. Is there a possibility that I will not be able to retain my priority date?

  4. Any other worst possibility that can happen ?

Thanks !