Nordic nRF24L01+ transceiver module question

I am trying to build a wireless thermometer using two (probably fake) nRF24L01+ transceivers. I planned to use their "Shockburst" features with acknowledgement and automatic re-transmit. I want to be able to send a command from the base to the remote sensor, and have the remote reply with the temperature data. I can't use the ACK Payload, because I have to do the measurement first. My question is, how long does the remote unit need to wait for the acknowledgement to complete, before turning off CE and changing to TX mode? I thought that 256 uS would be enough, but maybe I'm cutting off the acknowledgement before it's finished? I am not using the ACK Payload, and the Nordic docs do not really address this case.