The answer to this depends on which province you live in. It also depends a lot on the particular judge, because there’s a lot of discretion involved in deciding whether to award retroactive support.
The factors a judge in Ontario will consider:
1. Need of the support recipient and ability to pay of the support payor.
2. Improper conduct by the support payor.
3. Need to encroach on capital or incur debt.
4. Reason for delay in requesting an increase in child support.
5. Notice to the payor spouse of an intention to pursue child support.
6. Unreasonable burden on the support payor.
As you can see, a lot is going to depend on the specifics of your situation, especially why the arrears arose and the financial circumstances of you and your ex.