Once you fill out your details you'll pay NZeTA processing fee of $9 if you're using the app and $12 if you're using NZeTA website. You also have to pay an IVL fee of $35( International visitor conservation and tourism levy fee).
Processing time: Not more than 72 hours. Mine was approved within 48 hours.
Track your NZeTA status from the app
Validity: You can use your NZeTA for multiple trips of less than 3 months each to New Zealand within a period of 2 years after it's issued.
If you want to visit for more than 3 months you must apply for a visitor visa
New Zealand Visitor visa.
Application is fully online but you have to apply for your visa a few days prior to your trip.
More info on visitor visa issuing criteria and applications here.