so how can you get increase limit? how long should i keep that $750 rule for?
That "rule" actually isn't a rule. It really depends on what you are selling. Selling new computers at 20% below amazon gets that "rule" thrown out of the window along with your account.
Depends on your previous selling habits. IE if you sold 10k and pass a review you should be able to go to 15k-20k maybe more. But numbers wise no one will be able to give you an answer as each account is unique
As mentioned above, depends on category, items, price, many factors.
I have done over 5k on accounts each month and not faced a review ever, some around 1k in the first month and a review. If your selling something sketchy expect more trouble.