Dry hopping, after the main fermentation has died down, allow for around 4 to 7 days contact time.
Is the belgian ale kit already pre-hopped? If so you may find adding more hops may overpower the flavour of the final product. In my experience the thing that separates Belgian brews is the funky phenols/flavours coming from the yeasts.
The liquid yeasts are really good and very worthwhile adding to belgian brews (IMO dry yeasts have their place, but to get those cool belgian flavours you need a liquid yeast).
Dry hopping will add mainly aroma and a little flavour, but this may overpower the other flavours.
Try searching for a few belgian dark ale recipes and look for the common ingrediants (hop types and amounts) and use this as a basis for your recipe.
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