It of course varies by firm, but there are some general market trends. For example, many firms in NY pay lockstep bonuses -- i.e. generally everyone in class gets the same amount and it's not tied to billable hours. In DC, most firms give a bonus after around 2,000 hours billed, with the potential for more based on merit and of course more hours.
If the Chicago firm you're talking about is Kirkland, then this is good news (for bonuses, anyway). Kirkland gives very large bonuses accross offices.
It will be interesting to see if the past increase to 145,000/135,000 and the recent increase to 160,000 will affect bonuses.