From Birnam Designs Wiki

The sudo/gksudo timeout length can be modified by editing sudoers:

sudo visudo

/etc/sudoers must be edited with visudo, do not edit directly!

find a line with

Defaults xxx

or create a new one and append passwd_timeout=x with 'x' being the duration in minutes. e.g.

Defaults xxx,passwd_timeout=20

sets sudo timeout to 20 minutes