If I am holding an open long position and choose to assign the position, is it guaranteed that it will happen? Roughly how long does it take for the transaction to settle? Does open interest come into play?
Broker dependent. Some brokers charge the same as a stock trade. Some charge a line charge. If it is a retail broker, my expectation is that they charge a fee per strike. If it's 1000 contracts or 1 contract it will cost the same. You have to ask.