NPK Fertiliser and dogs

HI everyone,
I am very new to all this gardening business, having put down some fertiliser I am now concerned about allowing our dog out on the lawn.
Does anyone know how long after I have put down the fertilizer I should wait until allowing the dog on the lawn. The Fertiliser is NPK which I believe is harmful to dogs if they eat it?
It has been on the surface of the lawn for about 3 days now and its only slighty breaking up.
Any help would be great.

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