I'm not going to set a hard and fast rule, but I think DP has a point,

I would say ideally you should be around 20/20 if you do not have access to invisibility and have at least 10-15 move silent if you do have access to invisibility. This is more for you to consider whether you think your PC would be skilled enough for such, we'll handle the details IC, so even if your PC is no where close to this, but -thinks- they are, we'll deal with it IC

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Happiness is not a goal; it is a by-product.
-Eleanor Roosevelt.