Months/weeks can be deceiving. Someone can edit for 20 hours a week and finish an 80-hour AMV up in a month, while someone else who only can get 5 hours a week in is going to take "longer." Heck, some people have spent years on videos, and sometimes it shows, sometimes not.
Of course, the chances of creating something that LOOKS slapped together are much higher if you actually do indeed slap it together. On the other hand, if you spend the time to tweak and adjust everything, it's hopefully going to show. So, when in doubt, it's worthwhile to take your time.