Hello All
How quickly can the tone function be turned on and off as I am looking to design a SSTV/FAX encoder for arduino
Any help is appreciated
Best regards 
Jon
Welcome to the forum!
By leaving the timer running you can turn the output on and off using a single machine instruction (1/16 ?s). Add in the fact that you will very likely be coding in C++ instead of assembly and that an interrupt may interfere it is reasonable to state that you will be able to control the tone within a few microseconds.