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The Vicodin you gave her is 5mg of hydrocodone and 500mg of acetaminophen. Generally, the effects of this type of medication lasts about 4 to 6 hours. But, since she is so elderly, the effects may last longer.
As long as you can rouse her and she appears to be in a normal sleep state, it is likely the sedative effects of the hydrocodone causing her to sleep. However, if she is not able to be aroused, has very shallow breathing and a slow heart rate, she may be having serious side effects of the drug. It is fairly common for people with dementia to be intolerant of narcotic or sedative drugs. If you feel she is having this type of reaction, call 911.