In New Zealand the usage is much the same as Phil commented."When will you be free" is asking for an instant answer.
"When you are free" is asking them to notify you when they are free.
"When you have a free moment please let me know' = Are
'When can we arrange the dinner' = will be
"When are you free to take the dog for a walk"= will be
"Tomorrow when do you want to do the housework"= will be
"Let me know when you are free tomorrow to catch up"=Are