I find that in general it takes 2 to 2.5 hours for the ganache to set to the desired consistency. This may vary however depending on the exact type of chocolate and cream you use. I recommend checking after around 2 hours. You're looking for a spreading consistency, a little similar to Nutella or peanut butter. If it's still too runny leave it for a while longer. If on the other hand it's set too much, just microwave for 5 to 10 seconds until slightly softened again.