We already scheduled flights from the US to Milan next summer, but have now decided to immediately fly to Naples instead of taking the train, and need to schedule separate flights (it won't be a connecting flight). My question is how much time do I need to allow to deplane in Milan, get luggage, go through immigration, then check bags and board flight from Milan to Naples on a different airline? I don't want to be stressed about missing the second flight, but I don't want to waste too much time in between the flights either. Any suggestions are appreciated! Thanks.
Train time is around six hours, changing downtown at Milano Centrale. You really can't beat that by taking a flight. Also Air Italy gets very poor reviews, so you may be looking at Easyjet later in the afternoon. Get the Trenitalia app, tickets after retrieving your luggage.
Just be prepared-our last 2 trips through MXP this year it took over an hour and a half to get from the arrival gate and through PP control and then about another 30 minutes to retrieve luggage-if you don't land at lunch. This was because of the increased load of flights from Milan's other airport which is undergoing upgrading, and, given "Italian construction time" am pretty sure it will probably be the same case when you arrive. Then, after exiting and re-checking bags and going through security, wouldn't be surprised if you might need an additional 2 hours. The good news is there are plenty of very nice food and snack areas to chill out in at a variety of price points.
If you use easyjet, you change terminals ( extra 15-30 minutes) and have the easyjet policy of door closing 30 minutes prior to scheduled departure.
Allowing anything less than 4 hours looks to be unnecessarily risky.
I would be fully exploring the train option before making a final choice.
Do not overlook the Naples end of getting to your destination from the airport vs train station
Not knowing what time you land, allow 2 1/2-3 1/2 hours for a connecting flight. For airlines check Alitalia, Meridian and Easy Jet.