It often takes a long time. It shouldn't, but it does. Reimbursements often have to go through a good deal of bureaucracy. At my place, it goes from the department to the Dean's office to the Accounts Payable office, and needs approval at each step. Then, the Accounts Payable folks only cut checks on certain days.
So just call the department's administrative assistant, and ask, very nicely, if all of your receipts and paperwork are in order, and if there is anything else you should send.
Then, very nicely, ask if the administrative assistant has any idea about the potential timeline for reimbursement.
This is what I would do.
I'll second that things involving money often take a really long time to wend their way through the bureaucracy. Probably this is the case for your reimbursement check.