If you can be satisfied with the 30% – 50% that collection agencies charge, that can be very quick but the agency does not always collect the full amount of the debt. More effective – though slower – is to pay an attorney hourly to (1) assess your odds of collection, and (2) go after the client, first with a letter, and later with legal judgments and attachment of assets. I collected 100% of a $30K debt for a total cost of $2K, although it took a year.
If you need an attorney referral, let me know.