It is possible to modify the delay before encryption of the IP addresses (4 months by default) by defining this constant in your config/mes_options.php file (refer to the dedicated article).
For example:
// encrypt the IP addresses after 2 months
define('_CNIL_PERIODE', 3600*24*31*2);
To make SPIP never encrypt the IP addresses, you can define this constant as zero:
// never encrypt the IP addresses
define('_CNIL_PERIODE', 0);