I found this information regarding chest freezers but I would check the manual for specific recommendations from the manufacturer:
How Often to Defrost
The frequency at which you defrost a chest-type freezer will depend, to some degree, on your local humidity. In a dehumidified part of your house, or in an area where there is low humidity, you will likely defrost less often. In general, though, you should defrost your freezer every 6 to 12 months.
I did see that this model has a defrost drain conveniently removes water and saves you from straining to reach the bottom of the freezer.
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