I am with Amy on this one, it is way to early to ask how long.... You will drive yourself nuts.
Since you are in the middle of the rad treatment take it as it comes and be prepared to deal with the side effects. Yes it may be miserable and probably will get worse before it gets better...... BUT it will over soon and later it will get better. Tons of people have gone through this.
Re sprays:
I have not used them but people use soda/salt rinses that you can tolerate even when other stuff hurts. You can change the composition (lower or leave out the salt) if that bothers you. Also what helped me at the end of the rad/tx when eating was becoming "unpleaseant" was viscous lodocain. You can dilute this a bit (if necessary) and swish it around your mouth... then you have some time (1/2hr?) to eat and talk.. Watch out for development of thrush which often happens because your immune system is in thr crapper. (It makes things more painful, and needlessly so).

Best!
Markus