All logs of dpkg are saved at /var/log/dpkg . The thing is if you are on Debian testing or/and Unstable/Sid the churn of packages are going to be much more than in a stable release. Now while in a stable release the churn is not so much and 7-8 log files should be in a position to serve . But in testing this simply proves to be too little.
For instance at my own end -
┌─[shirish@debian] - [~] -  └─[$] zgrep " install " /var/log/dpkg.log.8.gz | tail -1 2016-05-31 12:26:29 install gnuplot-qt:amd64 <none> 4.6.6-3
Now while this is somewhat useful, if I want to go back till a year, all of that info. is lost.
How can I prevent it from happening in the future ?
It would be better if the change were only limited to dpkg logs rather than all the logs.