Thanks Robin2 for suggestion i have reported it to moderator
and as you suggested i think it is best to keep the master as listener and slaves to send only when data has to be send there is no need for setting specific time interval for sending data as it will have to wake up a slave for sending data at a specific time, instead of that only to send data when there is an occurring of an event.
i wanted to know whether are there functions in nrf24l01 library to start a data transfer and to stop data transfer or as it is based on pipe can we open a pipe for certain duration and close it again so that we shall send message to master only when a event has been occurred.