How long will it take to enroll in the iPhone developer program after paying the $99?
Is there anything to do after paying it? (I mean paperwork, faxing or sending physical stuff)
How will Apple pay your revenues? (In what form and intervals, and what will they do if you are not in US?)
About 1 week after the $99 - perhaps a little less or a little more.
Yes, you have to manage certificates and provisions. Also, you have to submit your contact, tax, and bank information. Nothing complicated though - just the standard stuff.
Apple pays by depositing funds into your bank account and providing a detailed statement of your sales.
You also might have to fax them a copy of your passport/ID if your contact address differs from the creditcard billing address.