i think if you leave befor the last ship is dead or befor the timer runs out u dont get it... happen to me last night

 I dont think there is a time constraint. A win is a win 10 seconds in or 10 minutes later, from what i've observed.

took me 3 wins to get 1st box, then had to get another 3 for second box, and didnt get a box, so im getting a box for every 5-7 wins >.<

There is no bug in the boxes. I opened all 4 of my boxes today with no problems, or issues. It doesnt like you. Also not all wins count. I have had a battle, to where a win did not count. Already a post about the win count. Can someone lock this. 

http://fm.en.kupaisky.com/Community/Forum/View.aspx?num=11855&searchtype=0&pagecount=0&searchvalue=&sort=5&category=C01&page1=1

Also, you already posted about this on the other post.