If you can convince a judge that the property is abandoned then you can get rid of it. However, there is no set circumstance or time limit that conclusively establishes abandonment. The Court needs to believe that she has made a decision to abandon her property. If you throw out the property and she decides she still wanted it, then you could be sued for conversion or civil theft (no good deed goes unpunished).
This is a very difficult situation. You need to give her a clear deadline to remove her property. You also will want to take pictures and make a good record of what property she has at your place so that if you do end up in a legal battle over disposing over her property you only have to argue about her property, not that rare Picasso she suddenly "remembers" leaving with you. She was storing the property in your garage and when you move out of the property there should be no obligation for you to move her junk with you, but until you reach that point you are in a sticky situation.
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You have become the "bailee" of the friend's stored items and thus have a duty to make sure that they are not destroyed and that they are kept in good car. So, put them in a storage unit - either a mobile or stationary one - and charge the storage costs to the friend. You do not have the write to destroy or throw away the items.
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I would start by sending a certified letter (copy by regular mail) that you will store the items for 30 days and if she does not retrieve them by the deadline that they will be deemed abandoned.