Just finished up rebushing the keys on my 1918 Hobart M. Cable "H". I have a quantity of hide glue that is now gelled in its covered jar - it has already been re-warmed twice.

How long can it be kept in this state and still be usable? And how should I store it.

TIA...

Follow your nose!

I've had some glues mold in a few days and some last for two weeks. A tech suggested using a drop or two of camphor oil so the oder isn't objectionable and it keeps the mold away longer.

Gelled hide glue can be kept in the fridge for some time, perhaps a few weeks. Why don't you experiment and try putting some in the freezer for two weeks, testing it afterwards, and posting results to the list?

Here in the N East, hide glue gets moldy after a couple of days.

Older glue becomes a darker brown color. I don't like working with it at that point. It seems less tacky in a way and and less forgiving.

Best way to know is to try it.
I like Jurgen's recommendation.

There is a chapter on hide glue in "Piano Tone Building" Recently edited and released by Del Fandrich. There was very good information on hide glue.

As Jurgen said, it can be kept in the refrigerator. It will start to look nasty after a time. You will know. I would only use the stored glue for things like keybushings that don't require as much strength. If hanging hammers, I always make up a new batch, because it's a more critical application.