Hey Nicky,
there are variables, but a good way to start is to always leave the amount of time for breathe-up the same as the last hold.
So, if you held your breath for 2 minutes, then breathe up for 2 minutes after, then hold again for hopefully longer, say 2:30, then breathe up for 2:30, etc, etc.
A good technique is to DECIDE ahead of time that you will hold your breath for a certain amount of time, and that nothing will stop you (within reason of course).
For instance, one day I DECIDED that my first hold would be 4 minutes....and guess what? That's right, 4 minutes accomplished, because I made up my mind that it would happen.
So think about your goal, imagine seeing yourself do it, then do it!
Make sure you are relaxed as possible during the breathe ups, and try this:
when the 30 second countdown comes, hold your breath for 10 seconds, exhale and breath 3 quick breaths, hold for 10 seconds, then exhale, 3 quick breaths, then do your big breath-hold. Let us know how well you do....go on my "Static Challenge" thread under Freedive Training and get in the game!
Cheers,
Erik Y.