Rover mechanical top speed = 180 meters/hour
Rover speed with hazard avoidance enabled = 36 meters/hour
Expected top travel distance per day = 40 meters

Distance of hills = 2-3 kilometers

So the hills are either 50-75 days away, 7-10 days away, or about 2 days away, depending on how you look at it. But the 50-75 day figure is probably the closest to being accurate, and, as someone else pointed out, it also depends on how many interesting things they run into on the way.

Keep in mind also that the mission success criterion is a total travel distance of 600 meters, so Spirit would be well out of warranty long before it gets to the hills. But we'll see - NASA spacecraft tend to last well past their "sell-by date," as a rule.

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Everything I need to know I learned through Googling.