The answer, quite simply, is very little. Illinois no longer requires that you prove grounds for a divorce and is essentially a no fault state. You no longer need to show adultery, substance abuse or abandonment to be granted a divorce. Instead, the person filing for divorce must show only the irretrievable breakdown of the marriage.
In an uncontested divorce where both parties agree to the terms, there is no required waiting period. If you or your spouse contests or challenges the terms or the divorce itself, then there is a requirement for a 6-month separation period. This separation does not mean you must live at separate addresses, but that you are living separate lives.
The reason for the divorce (infidelity, disengatement from the relationship, a drinking problem, etc.) has very little effect on the financial settlement of the case although those things can have an impact on such things as parenting time and child custody. Division of assets and debts and alimony/maintenance will typically be determined without regard for whether one spouse is more at fault for the divorce occuring.
If you are at the point where a divorce is necessary, feel free to contact us at (847) 599-9101 for a free initial consultation.