Legal Question in Real Estate Law in Illinois

There is no established period. The parties negotiate the contingency date as part of the contract. If the buyer has been pre-approved, many good lenders will get a loan done in a few weeks. It is common to see a contingency date approximately a month out from date of contract acceptance, but there may be reasons that the buyer would use a longer date and a seller may want to have an understanding of the reasoning before agreeing to too long a date.

I am guessing you already have a purchase agreement, and the lender is taking longer than desired. In that case, it would depend on the terms of the purchase agreement and the seller's patience. If you do not already have a purchase agreement, then the answer is, whatever the parties agree.