This is a very difficult question to answer. The reason is that it depends on your hardware, your network connection, the size of your existing Raiser's Edge database, the breadth of the data you are importing (how many fields spread out over how many areas of RE) and possibly (if you are using an web-based data source such as Eventbrite), your bandwidth.
There are several stages to working with Importacular which may take time.
1. Retrieving data from the data source.
2. Looking up records in Raiser's Edge.
3. Importing the data into Raiser's Edge.
We did have a file of 12,000 constituents that we imported. The file was simple containing first name, last name, street, city, zip code, state, country and an email address.
Stage 2 depends on the number and complexity of criteria sets that you are using. Our test case only used one criteria set to look up by email address. This stage took 20 minutes to complete.
Stage 3 took us 30 minutes to complete. In this example all of the constituents were created. There were no updates. We do not think that this ought to make a difference to the overall performance.
Other factors that can affect your speed include your Criteria Sets (particularly if limiting to a certain query) and RE security settings (such as those limited by Constituent Codes).