Assuming:
1) You can use LONG variables
2) You are looking for the time of each period
Code:
Freq VAR LONG  ' Length of time for each period

SELECT CASE x
   CASE = 1
      LCDOUT "001.000 Sec"
   CASE <=1000
      Freq = 1000000/ x
      LCDOUT DEC3 Freq/1000,".",DEC3 Freq," mSec"  
   CASE ELSE
      Freq = 1000000000/ x
      LCDOUT DEC3 Freq/1000,".",DEC3 Freq," uSec" 
END SELECT