Ill. Admin. Code. tit 86, § 950.450 - Limitations

a) No notice of proposed assessment may be issued with respect to a calendar year after June 30 of the fourth subsequent calendar year. (Section 85(f) of the Act)

b) No claim for refund may be filed more than 1 year after the date of payment of the amount to be refunded. (Section 85(f) of the Act)