Is it common to have a droopy eyelid after retinal surgery and will it heal?

I can't give you an actual incidence but ptosis after retinal surgery is pretty common.  Typically, there is a slow resolution of the ptosis over 6 months.  For this reason, it is best to allow this to heal on its own.  After 6 months, if there is residual ptosis, surgery can be used to address the issue.

Any type of eye surgery can cause a ptosis.  To do an eye surgery such as cataract, glaucoma or retina, a speculum is placed between the lids to keep the eye open.  This can stretch the muscle and you may have a droopy lid post-op.  It can take up to 6 months for the lid to return to normal.  Sometimes it doesn't and you have to have a surgical ptosis repair.  

When the eye lid is stretched as it would have been for you eye surgery the junction of the levator muscle and the tarsal plate might have been stretched or separated. If this is the case you will need surgery to reattach them correct the drooping (ptosis) of your eye lid. Give it a few more weeks and then check with your ophthalmologist who may be able to fix it or refer you to a Board Certified Plastic Surgeon or Oculoplastic Plastic Surgeon.