You can put in more if you will be using it again fairly soon - maybe within a couple of months. The important bit is to not put in too much because that leads to all manner of problems when it overflows and gets all over the place.

In any reasonable amount of time it just sits there, unless you handle the loco and turn it over, in which case it will run out on the outside and make another and different mess.

And you should not power the smoke generator when it is dry, which at least in the past would toast them for sure.

Thanks for the replies.