In the legend of drizzt how long is a tenday is it ten days or longer?

it's a week

1 ten day = 1 week

5 ten days = 5 weeks (etc.)

According to the Forgotten Realms Wiki a tenday is actually ten days long. "...divides the thirty day months into three blocks of ten days each, or simply put a tenday" It can normally be used with the Earth term 'week'. See the related link for more information.