The term of the agreement should be as short as possible. A good outside time limit is six months. A longer term makes the agreement look like an employment agreement, not an IC agreement. If the work is not completed at the end of six months, you can negotiate and sign a new agreement.
The date the agreement begins can be the date you sign it or another date either before or after you sign.
Suggested Language
Term of Agreement
This Agreement will become effective on _________, 20__, and will end no later than ________, 20__.