chkuser configuration is not configurable on the fly. Configuration settings currently in use are documented at /var/qmail/doc/chkuser_settings.h.
To tailor your chkuser settings, use the following procedure (provided by Nick):
# rpm -Uvh qmail-toaster*.src.rpm
# cd /usr/src/redhat/SPECS (assuming you are using CentOS 4)
edit qmail-toaster.spec. find 'sleep 5' (should be line 606). change 5 to 300 (5 seconds to 300 seconds). save.
# rpmbuild -bb --with cnt40 qmail-toaster.spec
(use the appropriate --with value for your distro)
When the build process pauses, open another terminal and edit /usr/src/redhat/BUILD/qmail-1.03/chkuser_settings.h to your liking. You have 5 minutes (or whatever you changed the sleep parameter to). Save your changes and wait for the 300 seconds to expire and watch it build.
# cd ../RPMS/i386/
# qmailctl stop
# rpm -Uvh --replacefiles --replacepkgs qmail-*.rpm
# qmailctl start
That's the procedure for now.
An easier method to the above is to open two shells, start the build in the first one and ctrl+S when its preparing to compile, then make the modification to chkuser_settings.h, and then just do a ctrl+Q again to carry on.