Is there a time limit for migrating? When is the dead line?
There is no time limit. You can safely wait to migrate later.
A Recent governance proposal was voted in to give the migration a dead line. The voted dead line was October 10, 2021 -- the migration contract is planned to be closed at that date. However, the vote has been since contested in community discussions and a new deadline for the migration will likely be proposed and voted on.
Ethereum fees are expensive :(
Ethereum fees (gas) are sometimes expensive, and sometimes it's cheaper. It's often cheaper at different times of the day. Often cheaper on Thursdays and Saturdays... You can wait for gas to get cheaper, and then make your transaction.