Characters Create Dragons and Dungeons Falruns The Watcher from the Aether says...
Typically, the way I do it, is that you cast a 'like spell' on the staff and each level of spell restores one charge.
So, if you cast Cure Light Wounds on it, it restores one charge. If you cast Cure Critical Wounds on it, it restores 5 charges because it's a 5th level spell.
In relation, if we were talking about a wand of fire, a fireball spell would restore 3 charges because it's a 3rd level spell.
Also, since they're 'like spells', it would do no good to cast fireball on a staff of healing and, likewise, it would do no good to cast any type of healing spell on a wand of fire.