It depends what you're looking for, but afternoon although extremely busy is in my opinion the best time to go. If you don't want the hassle of a huge crowd go earlier so you can get through easier without having to fight off a huge crowd.
We enjoy going for lunch. Try the cheese samples first then head downstairs for a nice selection of foods. Then finish off with some ice cream!
I would say the earlier the better, just to beat the crowds. We went in February, and around lunch time it filled up.
This time of year, it probably won't be too crowded, so you should be ok anytime. In the Summer, I expect going early would be good.
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