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231 helpful votes You seem a bit geographically confused! this is hte Hastings (NI) forum;your title says "Nelson" but you want to visit Marlborough!!
Blenheim is not a very exciting town so I would consider 1day is enough for visiting the wineries, and then I'd move on to a more scenic areea such as Nelson or Kaikoura - but this depends on the rest of your itinerary and your interests.
Here's a thread I saved that was very detailed about wine tasting in Marlborough:
As Zhuhai pointed out (besides the geography lesson), Blenheim isn't all that scenic. If you do decide to go south to Kaikoura, there was an interesting treehouse accommodation that I wish I could stay at:
http://www.hapukulodge.com/kaikoura/
There actually is a winery district around Nelson. And as long as I'm on a roll with links:
Has Nelson moved?
I don't think we can be in Hastings as we are in brilliant sunshine, as usual. But really it would have been polite if someone had pre-warned us our entire city was relocating!