Some states have laws requiring that a case be brought to trial within a certain time (eg. 1 year from date of filing) - obviously that is not happening in your mother's case. I'm not an Illinois lawyer so can't say what your state's law is on that but generally speaking, the parties to litigation spend months sending out written discovery and taking depositions of parties, witnesses and experts in preparation for trial. Once all that is completed and any motions to try and end the case before the trial are completed (which can take months themselves because each side usually gets 30 days to respond to a motion by the other side) the court will set the case for trial. Some courts set a trial date as soon as all defendants have been served with the lawsuit and have answered the lawsuit and then discovery and law and motion takes place. The fact that the attorneys are getting the depositions completed means they are getting ready for trial or to file motions to try and end the case earlier on grounds that one side or the other is likely unable to prove all they need to prove to prevail in the trial. Check with an attorney in your state to find out what the court rules are on getting a case to trial. Busy courts can continue trial dates if other trials were set ahead of yours and the trial is not completed yet. Some courts will send the parties to another judge if their calendar is too booked. Again, check with a local attorney. Your mother should discuss with her attorney the likellihood of settling, mediating or arbitrating sooner than trial. This will depend on how strong a case she has. She won't want to suggest it to the other side unless they are in a weaker position than she is as it can be interpreted by opposing counsel as a sign of weakness in her case or that she is running out of money.
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Unfortunately the deposition is just one step in the process and does not necessarily indicate that your case is approaching conclusion. The attorney handling your mother's case should be the best source of information regarding the anticipated duration of your mother's case. Many times there is a case management order in place that sets out the time line for completing certain activities in anticipation of a certain trial date. That would be a good indicator of what kind of timeline you might expect. However, as previous answers have indicated, for a variety of reasons trial dates can be postponed or changed.