Hello Tot.  I am active in siege contests every month.  I will try to give some help.

For points, if you do 1000 sieges = 100k points.  (1 siege = 0.1 contest point).   But, most weeks,  you need to get 300k ~ 1mil siege points to get top 3 position.  For me, it takes about 30+ hours to do that. Not easy.   But, 100k points is usually enough to get top 20 or top 10.  Good enough  to complete your month - top 50 Chronos quest.

Siege rankings reset every week.  If you play one week,  it is best to recall your troops after contest ends.  It is easy - just go to 'garrison', click recall all units, done'.  That is why top players have many sieges, then after contest ends, they recall  and the  sieges disappear.  It is normal game play.

 But if you recall, you must wait minimum 1 week to participate again.  Why?  You would need to siege all your old sieges + add new sieges to get points.  That is too hard.  Just wait for 1 week or more, then start again.  After 1 week, your siege bank resets to 0. You can get new points and try again.  

Good luck!

-KJM