When are the commission rebates paid?
There are two types of commissions paid by Lenders - an upfront commission and a trailing commission. 
The upfront commission is generally received in the period 4 to 8 weeks following settlement of your loan. 
Trailing commission is paid monthly in arrears.
Be very wary of any 'cash back' mortgage broker who only pays you rebates once a year.  Not only are you taking a massive leap of faith that the company will still be in operation - your interest savings are dramatically reduced by delayed payments.