Some people add some Colloidal Silver (I believe I've seen this mentioned somewhere on the forum - topic, not articles) though I can't tell you how much but even with, I don't think it would extend the life of the colloidal gold by that much (maybe a week or two if you keep it refrigerated?).
If someone else has done this, please do chime in.
Obviously, the simplest way is to make it in small, say 250g batches and refrigerate it and plan on using it up over the next week or two. That is why I believe the formulas at least for using salt lake minerals gold chloride are for a batch size of 250g. Given how quick this method is, you can have more for a couple minutes work and a little elapsed time so its no big deal.