nrf24l01 frequency switch timing
I received a response from Nordic support. Exact text as follows:


  The nRF24L01+ can only change frequency in standby mode. It takes 130 µS from standby to active mode (RX or TX) regardless of you change frequency or not.
  
  This applies for the nRF24LE1 as well.


So as it turns out, switching from standby to Tx/Rx will also setup the RF frequency without requiring any extra time.