3 months, 6 months, 1 year [...]
I think 1 year of inactivity will be enough 
This is just a short note and I will return later to reply to earlier posts.
Sorry, but I thought this was clear from my post
To be more precise, this part:The event is related to the activity made by users during current year and not for the past.
I am sorry if the way I wrote it created confusion. By active users I meant users which were (and are) active during this year.
what if an inactive account wakes up during the competition?
This would mean that he, actually, is active during current year, right? And the contest addresses users with huge contributions made for this year. So, for example, if an old user wakes up this year, he can be voted, but he should also merit to be voted for something amazing done during this year. If he just wakes up but has no contribution -- as he just woke up -- he would not deserve to me nominated, right?
Anyway, these are very specific situations, which are also unlikely to happen, but even if they appear, we will treat each case based on its particularities.