I believe the condition under which the alternator only charges on deceleration is in Eco-Pro mode, so if you drive in that mode a lot, you would have to drive further to get the same amount of charge as with other modes. With a new battery, you could easily go 2+ weeks without needing a charge. When the car passes the 3-year mark, that interval could shorten, as our batteries have tended to last roughly 5 years. That's when my E92's battery needed replacing.
2013 F30 335i M-Sport 8AT/MPE/MPPK, 2016 228i M-Sport Cabrio 8AT