I usually allow 3 hours for US immigration, but I have done it in less than an hour before at EWR (carry on only). If you arrive behind a flight from Pakistan, India or some similar place then you will be in the line for a while so things are very open to luck.

The worst thing is being directed into a promising looking line that is only 2 people long, but then it transpires that both of the people in front of you are are illegally-immigrating Spanish-speaking heavily pregnant pirate terrorists who have recently visited a farm and once worked for the '38-'45 Nazi regime while laundering money and trafficking drugs between Cuba and China.

My 3 hour window is partly because I want to keep my US domestic upgrades which get very unreliable if you get bumped to another flight. As a BMI *G this won't matter to you. (And the *G status will help if you need to be rebooked on another flight due to a misconnect.)