It depends on what the medication is as to what happens. The normal thing for AZ is Donepezil (Aricept) in which case you won't see an improvement just a slowing in the rate of decline.
Other things like anti depressants can work quite quickly and can make a positive change for the better.
Not all people react to a medication in the same way, things that work well for some people are a disaster for other people.
All you can do is monitor it and be aware of the possible side effects.
Sorry I can't be more helpful but it's a bit of a lottery.
K