Cydia is a UI front end for apt-get, apt-get need to refresh its sources before it can let you know if any update is available or not, this doesn't happen unless you open Cydia.
As a workaround there are some Scheduling app you can install and then have a script call for a apt-get update in the background, but this have to be done on using command line, if you are confortable with that it is a lot easier to just open cydia once in a while.