going into flowering (help)

my plants are now aprox 2 foot tall and think its time to flower them do i need to flush all newtriants ot before i feed them the flowering food

if so how do i do this and how long does it take im using hydro set up

cheers

If its properly ready for flowering, don't worry about flushing nutrients I would say its better to leave them, but don't add too much either.
And for flowering depends, some plants may take 2 weeks, some 3 or abit more and also how you take care of them.