How can I estimate the speed of this code section for this microcontroller?
Since the code snippet you're interested in isn't big, you could disassemble your compiled code, look at all the assembly instructions and count how many cycles they need. You can find the number of cycles for each instruction in the datasheet.
If you have an oscilloscope, you can turn on a pin before the if statement and turn it off after your code snippet. (Using direct port manipulation PORTB, not the Arduino library function) With a scope you can see how long it takes to run the code. 
Use the micros() function in the Arduino library. Place one before and after the code snippet. However here you will have a couple of microseconds overhead since the 'micros()' has to run as well.
Use a debugger or hardware simulator that can count cycles. Put a breakpoint on the first statement of the code snippet and one on the statement after the snippet. delta_t = cycles / clock_freq (in line with Oldfart's answer)