There are too many variables in a restore to say "that you should experience a 2 x backup time for you restore time". Your backup is backing up 8k pages that are in use, but your restore has to account for the full size of the database regardless of used 8k pages. So, some of the questions to answer are:

1. Are you restoring to the same hardware (or very similar)?

2. Are you on a SAN and are you restoring to the same level of spindles (or SSDs)?

3. Does the restore DB already exist on the machine or will it create it new?

4. Is instant file initialization turned on on the restore machine?

5. How is the size of your DB distributed? MDF and NDF vs LDF (Log file can't use IFI).

6. Will you be restoring from a network share, local storage, etc?

The best way to tell how long it will take to restore is do actually restore the database. Good news there is a query that you can use to help let you know how long the restore will take once you have started it. I have provided a link to the blog post with the query:

http://johnsterrett.com/2011/08/29/how-long-will-this-sql-server-restore-take/