rpm -qa --last will return all installed rpm packages with their installed time. The last installed packages will be at the top of the list.
rpm -qa --last | less is will return all installed packages with their install date. Less allows you to scroll through the results.
rpm -qa --last | tail -n [lines] will return the last 5 packages. Replace [line] with any number you want, in my case 5 for file lines.
[[email protected] root]# rpm -qa --last | tail -n 5
termcap-11.0.1-17.1 Tue 09 May 2006 03:02:53 PM MYT
setup-2.5.27-1 Tue 09 May 2006 03:02:52 PM MYT
filesystem-2.2.1-3 Tue 09 May 2006 03:02:52 PM MYT
basesystem-8.0-2 Tue 09 May 2006 03:02:52 PM MYT
redhat-logos-220.127.116.11-1 Tue 09 May 2006 03:02:51 PM MYT
rpm -qa --last | grep [package name] will return the install date for a specific RPM package. In my case the apache web server [httpd]
[[email protected] root]# rpm -qa --last | grep httpd
redhat-config-httpd-1.1.0-4.30.2 Sat 29 Mar 2008 09:03:40 PM MYT
httpd-2.0.46-70.ent Sat 29 Mar 2008 08:58:19 PM MYT