I want to reserve an app name that I intend to build out over the next 90 days, how do I do this in apple\'s web developer portal?
This resources implies that app names no longer expire.
90 days you say? This wouldn't be the first project that was supposed to take 90 days but needed more.
Once you fill in the information in itunes connect you have 120 days. After these 120 days your app name will become available for other developers. Yes, others, you can't take it again.
So you better make sure that you have your app ready for sale in 120 days.
You should read the itunes connect developer guide. It explains the process in detail. It's too complicated to answer this in a 5 minute stackoverflow answer. But the others outlined it, and I just wanted to mention = the 120 day limit. Happy squatting.