You will need to review the labor laws in your state. However generally accepted business practice is 40 hours is a full time Salaries employees scheduled time. Even though it is normal that a salary empolyee, like yourself is expected to "chip" in and work more hours when needed. Some companies also work with as low at 35 hours. It all depends on your company. It would have alot to do with the work you do and what is expected.
I am reasonably sure that you will be fine with 40 hours as the standard. As your recovery improves I would test the water with more hours until I am able to return to full duty. Keep in mind there will be days that you over do it. It is speaks volumes of you and your character as well as encourage others that are working to cover your job.
Good Luck and I hope you are able to have a 100% recovery and return to your job full bore.

