I also follow the "one year rule" with most client projects.
For documentary and personal stuff I normally do hard drive copies and if the material is important and needs to be stored longer, an additional LTO copy for about 10 years of storage or more. for documentary stuff that may include everything even if the project is done: raw materials, offlines, edit and fx projects, finished masters, etc. Future proofing edit projects is very challenging even in two year time scale so it is best to simplify the project and export xml out of it so that it can be read at least to some extent with future programs. and all finished vfx rendered to tiff or dpx and audio to wav or aiff, you may not have the plugins anymore next year if you need to open the project for some reason.
if you need to store client projects for longer than needed for editing purposes they should pay something extra for it for sure. for very small single day shoots I may additionally dump all the material to separate LTO tape or even BluRay disc if very small amount of material and let them store it as a extra backup. it is very typical that clients store the material on hdd's carelessly and will call you after a year or two to beg a copy of the raw materials they have lost. it may be easiest to give them an additional backup copy in a format they can't easily destroy or erase and then just erase all your own copies and never worry about the materials anymore