Hi, Karen.
It's hard to give you an average percentage or standard measures.
Some things that can improve your response rate:
Use a variety of question types.
Don't make the survey too long.
Break up your questions into more than one page.
Offer an incentive for taking the survey, especially if it's lengthy.
Ask questions geared to your audience. E.g. Don't ask older women about their bodybuilding routine.
If you have an active survey, look at the partial responses and take notice if there is a particular page in your survey where people tend to drop out. This is a clue to take a look at what you are asking there.
I hope this helps.
–Cheri