I’ve not tried using evaporated milk to make a ganache. I suspect that once opened evaporated milk will spoil as quickly as cream.
Do a simple experiment. Take four sterilized cups, half-fill one with fresh cream from the fridge. Half-fill the second with fresh cream brought to the boil. Half-fill the third with fresh cream ‘home’ pasteurized, ie raised to 72° C for 16 seconds. Finally, half-fill the fourth with evaporated milk. Leave in a warm room and check twice a day, tasting each until you think it unwise to taste anymore.
Shelf-life is dependant on: 1) the bacteria and spores introduced into your confections from the ingredients (particularly cream), air, hands, utensiles; and 2) the ‘water activity’ of you final preparation. Water activity in turn is dependant on i) moisture, ii) binding of that moisture (ie by sugars, sugar alcohols), iii) temperature.
Better to bind as much of the water in your ganache as possible. You can do this by: a) adding sucrose b) by adding glucose, fructose or invert sugar c) by adding sorbitol d) by inverting the constituent sucrose of your chocolate. Which will taste sweeter? – d) will produce a less sweet ganache than c) which will produce as less sweet ganache than b) which will produce a less sweet ganache than a) for the same moisture and water activity.
From my experience, to achieve a 3-6 month shelf life for a ganache you will need to either reduce the moisture to too low a level or reduce the storage temperature to near zero or below.