A question about serial communication and received buffer
If I were you I would try it out. It looks to me like a case where it is hard to determine afront what will actually happen. Let it run and try different delays for testing.
I think the hardware wouldn't discard the buffer that fast. What software are you using and what hardware/software is your slave would help a lot answering more precise I'd say. :-)
In a SPI-Interface it can f.e. happen that if you do not pull up csn after sending your data, the buffer will not be emptied at all (until you shut down the device since the memory is volatile). I'd assume a similiar behaviour for UART.