[TYPO3] raising cache-table expire time

[Matyi Gábor]

There is a TSFE config variable: message_page_is_being_generated His comment 
is:

Normally when a page is being generated a temporary copy is stored in the 
cache-table with an expire-time of 30 seconds. 

My question, how could I raise this time f.e. up to 60 sec

I get often a server error page  "premature end of script header: index.php" 
message, after 30 sec, if the imagemagick convert couldn't process the images 
on the FE.

I would happy If anyobdy at least could help me to solve this problem.

Thanks Gabor Matyi