How long does it usually take to get the pay by mail invoice after using a tollway?

Used the express lane on 45 back in March and still haven't gotten the invoice for it. A friend of mine said it can take a while, but surely not this long? I tried to check on the HCTRA website but it just says no transactions found for my license plate number. Did they somehow miss me? Really don't want to get into a situation where I've got an unpaid invoice floating around stacking up late charges.