That certainly makes sense, so an assessment of 'likelihood' would apply. There are three possible approaches.
1. Accrue 100% of commissions as the hope is that all clients pay within 90 days.
2. Accrue a percentage of the commissions based on a historical assessment of the number of clients that pay within 90 days.
3. Do not accrue anything if more than half of your clients pay after 90 days.
I think taking an approach based on how often commissions are actually paid out would give you an idea how to best approach this situation.
In my experience, I record 100% of commissions as an expense when the invoice is issued. Any invoices not paid, I then reverse the commission once it is certain that the commission would never be paid.
I hope that helps.
Feel free to contact me if you wish to discuss further.