I want to change sudo timeout, so I edit sudoers via visudo (as adviced in many tutorials online) to the following form:

Defaults env_reset , timestamp_timeout=20

root    ALL=(ALL:ALL) ALL

%admin ALL=(ALL) ALL

%sudo   ALL=(ALL:ALL) ALL

Defaults:my_username timestamp_timeout=10

the problem is, that nothing works and it seems I have sudo timeout=0. This is my output from uname -a:

Linux comp 3.0.0-12-generic #20-Ubuntu SMP Fri Oct 7 14:56:25 UTC 2011 x86_64 x86_64 x86_64 GNU/Linux

UPDATE

What I've noticed it's only happens when I use fish (aka user friendly shell). So problem probably with fish itself not with sudoers file.

SOLUTION
The problem was with my shell fish aka user friendly (not very friendly this time, isn't it?) shell. For some reason it seemed that sudo command each time went from different sessions.
Adding
Defaults !tty_tickets
in sudoers file (as described http://us.generation-nt.com/answer/bug-598567-sudo-requires-reauthentication-each-use-ignoring-time-stamp-help-200510161.html ) solved the problem. See the link above for the addition information.

Thank you all for your attention without it I could not figure the thing out.