it depends on how often you practice this accent and if you are fluent in whatever language the accent applies to or if you are a candidate for that particular accent.
Like, if you were a person from Japan and you need a Japanese accent in English. Then you wouldn't really have to know the language fluently.
It really depends on the circumstances and how often you practice if you must.
There are some actors and actresses out there that are really talented and can pick up an accent perfectly within months.
well with me it doesnt take long ... my mother in law is from Sweden and I already picked up the accent ... I have to stop myself from copying her and being rude!
It you pay close attention to the way they pronounce words you will catch it pretty quick ... and if the person has a thick accent you will have to pay close attention to figure out what they are saying ...