Are you saying after a fresh install, you defragging after ever program your installing? No need to do that. Usually after a clean install I install all the programs and drivers then I will run a defrag. Then ever so often after that depending on how much the computer is used.
It's a good idea to defrag before you install things too. that way the free space that the files will get written too will not become fragmented quickly when the files are on there.
its' good insurance to defrag prior to burning a DVD. I find I have fewer errors that way. I've wasted a lot of disks, an expensive way to learn. Not after each installation of software though, that's not necessary. I assume you are running windows since lenux has no defrag feature.
I think the frequency of defragmentation can not only be judged by the time but also rely on the size of fragment files in the disk. If there are excessive fragment files, it can slow down the read-write speed of files and weaken the computer performance as well. This is the intangible loss of your computer.