Hi,
The short answer is that it can take a while. Sometimes months. It all depends on the availability of the item from publishers. Older books can take longer as publishers these days do not like to hold onto stock that hasn't sold. If the book is older than 6 months, meaning it was published more than 6 months ago, you can see if it's available via Interlibrary Loan, and have it within a matter of days.
http://www.ebrpl.com/LibraryServices/InterlibraryLoan.html
Since acquisitions emailed you, it means that they'll reserve the book for you and notify you that it's available for pickup as soon as it has been purchased, processed, and ready for check out. There's no reason to keep checking back. You may see a record appear in the catalog with no copies attached This is a kind of placeholder for when the item arrives. It sometimes confuses people as to why the record is there but there are no copies.
If you have any further questions, please feel free to contact us.