Normally when a good cure is not achieved within the twenty minute window it is because of the amount of hardener used is too little, this can result in the product staying soft not going off at all.

In these cases the chemicals in the product will emit a strong smell and will not ever "dry"; there is no value in leaving the product in the fill if it isn't hard within twenty minutes.

Note that after a long storage time the hardener paste will separate somewhat and the first squeeze will give a very runny watery liquid, the hardener paste should come out of the tube like a tomato concentrate (or other paste like food). The watery liquid will not cause the bog to harden.

To achieve the best result, it is suggested to weigh the components. The best ratio to use is 1 part hardener to 40 parts Builder's bog when weighing. If you don't have access to a kitchen scale, you can use a visual guide of around 1 pea sized drop of hardener to a golfball sized dollop of the Builder's Bog. 

Another aspect to consider is that too much hardener will cause Builder Bog to harden only to a rubbery consistency (not fully hard like wood).