It varies according to the model and version of IOS.
Also if your operations staff isn't good about keeping the flash cleaned up there acan be lot of unimportant files there - old images and configs, crashdump files etc. that have no useful purpose.
Better is to have a network management system that is doing periodic updates of all of the relevant configurations, including IOS images where applicable. If you're running current IOS it can always be retreived from cisco.com as well.
I would ask though, if you are planning for Disater Recovery (DR), what is the scenario where you would have to recover a device but don't have the operating system? Whether its at a DR site, from local spares or an RMA from the vendor, the IOS should already be included.
