the MySQL library is not scheduled to be removed until PHP V6. The time frame for when they will be obsolete depends entirely on the host and when they force you to use a PHP version that does not support the MySQL library.
As an Example, With 1 and 1 hosting, I am able to use either PHP 5 right now, or I can select to use PHP 4 if I need to for older applications.
I'm sure when PHP 6 is released I Will at east have the option to use PHP 5 which will support the MYSQL library or use v6 if I Wish,