Thanks a lot David.
So all advertisement will expire after a month or so... I would probably be deleting less than 1% of the index on each bulk delete request... but after around 1 month, 100% of Index is deleted as all advertisements eventually expire. From your explanation, it sounds like I should be fine with just deleting them as they expire (probably combining a couple of deletes in one bulk delete)? Is my assumption correct that ES merge will happen on a regular basis (say everyday) and therefore this approach could work?
I really like your second solution. the problem is I would always have to search the two most recent indices. for example an advertisement created 29th of March, is still active on 6th of April, so I need to search both March and April Indices. Do you think this would work?