I chose 500+, but it really depends on the gun. Some need to be broken in, some don't. For revolvers, 200 rounds of carry ammo is generally adequate. The improvements in modern manufacturing in QC and CNC machining means that most name brand pistols of modern manufacture are pretty reliable right out of the box. It takes about 500 rounds for me to trust them, and most importantly, the magazines. Think about it: on a semi-auto, which part is most likely to be defective, the $500-$800 gun or the $19.95 magazines?
For semi-auto duty rifles, I like to put about 1,000 rounds through them for familiarization and break in. I also shoot and zero them with different ammo at different distances so I'm totally familiar with their ballistics. For pump shotguns, 500 rounds is good. I like to give them a good wringing out to get them settled in and find out how they pattern with each kind of ammo.