Adsense after x amount of lines / paragraphs?

Hi there,

i need guidance in the following scenario,

im trying to get my adsense code to repeat itself (in loop) 3/4 times during the actual content of a post… (adverts are text based)

i would love the code to display itself according to the length of a post, this way the adverts will always be displayed equally spaced from on another no matter the length of a post.

alternatively it might be easier to have the adsense appear after x amount of lines / paragraphs within the post? either way though im sure you guys will be able to tell me what is and isn’t possible…

the adsense will be appearing on all sub blogs in my site so i need it hardcoded in the singles-template or as a sitewide plugin.