Popping forward 30 seconds...HELP

Here is my deal.  I can pop (fast forward) 30 seconds at a time by hitting that pop forward button.  I can also go back 10 seconds by popping it back.  My question is, can I change those lengths?  I want to be able to pop it forward 20 seconds instead of 30 seconds and I want to pop it back 20 seconds instead of just 10 seconds.  Please help as I've looked everywhere and can't find how to change these time lengths of the pops.

Vaughn