Info on Salary and work permit

I need two input from your expertise;

I have recently got a job in one Belgian Bank and below was proposed salary;
I am here from India and with my wife and no kid;

Salary Monthly- 4.300 € and Flex budget - 2.187 €

Gross around 6600 € per month

1. Can you tell me how much approx would be in Net in hand and what’s this flex budget;

2. Also since I came here last year aug with work permit type B from an Indian IT firm, I need to change my workpermit to new employer;

what needs to be done and How many Year I can stay in Belgium with B type workpermit and can continue the job;