Hi! It depends a bit how you apply the crumb coat. You can either put a very thin first coat (called the crumb coat, because it traps crumbs), chill for 30 minutes, then apply a second, thicker coat, which you would let set for another 30-60 min before applying MMF. This is usually recommended when you are making a buttercream cake (with no fondant), so no crumbs can be seen.
Since you are covering in fondant, you can directly apply a single thick cover of buttercream.
Which buttercream are you using? If you are using a crusting buttercream (butter with double amount of icing sugar), it’s usually enough to let it crust at room temp for about an hour, or chill it in the fridge for 30 min before covering in fondant.
If you are using a softer buttercream (like Swiss meringue BC), then chill it an hour in the fridge – however be careful as the fondant can become sticky when laid over a cold cake.
Masking with ganache is recommended because it becomes very stiff at room temperature and you have a really stable firm surface to polish the fondant on. However since you are a beginner, maybe best to stick with stiff crusting american buttercream.
Good luck and let us know how it goes!
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