What's the strongest magnetic field safe for someone with a pacemaker ?

Not much: People with electronic pacemakers should avoid any magnetic fields, in particular MRI scanners and power generators. Your cardiologist can give you better information about smaller magnets, but those from car speakers and such are safe. Remember that magnetism drops off quickly with distance, so even a powerful magnet is ok at a distance.

Consult with manuf: With the new pacemakers being better please consult with the manufacturing company of yours the detailed precautions you need. Worst scenario contact the manufacturer for detailed instructions.