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Hi jeff
When people talk about adding more data, typically they'd just be referring to more subs (sub-exposures) on the one night. That is, instead of 3x5 minute exposures (for example), do 10x5 minute exposures.
Generally i'd say an hour of exposures minimum for a good signal to noise ratio on a deep space object.
There are people that do add data like you're describing - for example, going out the next weekend and grabbing another hours worth of data and adding it to the previous weekend. It would be unusual though for someone to add to data captured a year ago.
When adding data from multiple sessions, definitely the camera orientation is important, as is the focal length, sky conditions etc.
People with permanent setups (and camera permanently attached) have a huge advantage in this.
Otherwise, you can use a program like registar to align your images - it will take rotation into account and deal with it. Deep Sky Stacker (DSS) will also do it fairly well.
Hope that helps.
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