Unfortunately, there is no way you can know when the key has stopped. So, you have 2 alternatives

1 - Let each command run for a small fraction of time before going back to prog
example
Code:
FRT:
     HIGH 4
     HIGH 6
     pause 100
     LOW 4
     LOW 6
GOTO PROG
2 - Put a timeout. So, if Serin does not give you a character, turn off all outputs
SERIN PORTA.0,T9600, 1000, Timeout, RX

Timeout:
Code:
   LOW 4
   LOW 5
   LOW 6
   LOW 7
goto PROG
This is from the manual
An optional Timeoutand Labelmay be included to allow the program
to continue if a character is not received within acertain amount of time.
Timeoutis specified in 1 millisecond units. If the serial input pin stays in
the idle state during the Timeouttime, the program will exit the SERIN
command and jump to Label