As my pack of Denny’s Favorite was leaking and swelling I decided to open it and make a starter. I made it from the normal 100 grams/1 liter DME. Now my question is: how long can I keep the starter in the fridge, in a mason jar? Would it be the same as when I recuperate yeast from a real batch?
PS the starter is tasty! 
Since I brew smaller batches, I often have some starter left over if I make one. Just as you mentioned, I treat it the same as yeast harvested from any other beer - stored in a mason jar in the fridge. I re-prop it if it’s sitting around longer than a week or so.
I frequently make an extra 500 mL of starter when propping up a batch of yeast for a brew day. Prior to the starter being cold crashed, I will pour off that extra 500 mL with suspended yeast into a sanitized mason jar and have stored that cold (33-35F) for up to 6 wks with no issues. Of course a starter has to be made from the saved yeast. I would bet with proper sanitation that this yeast could even be stored past the 2 mos mark with no ill effects.