When the client publishes a gig, they try to be as accurate as possible. Sometimes small adjustments are made from what was first stated in the gig's job description to what appears in the contract. It is always the contract that applies.
It may also happen that in the gig's description it says that an assignment extends over 12 hours, but that it is sent to multiple talents and that assignment is divided into two six-hour sessions.
If it is a gig that is published and you can only work part of that assignment, you can write a note about it when you click on the application.
What matters is the details of the contract so read carefully before accepting.