H1B duration after 140 approved in different job fields

[ask_dr]

Hi, i worked on H1B for 4 1/2 yrs in IT. My 140 is approved but I changed fields. I am about to start working on H1B again in a different field. Will I get 3 yrs on H1b or 1 1/2 yrs initially and then an extension for 3 yrs based on my I 140? Can I even use my 140 to get extension for H1 in a different field? Please clarify. Thanks.