Original windows in older houses can last 100 years and then, with rehab, last another 100 years. An older window with a good storm is a very good match for a new window in terms of energy efficiency. I think replacement windows are one of the biggest scams of our lifetime. Unless you get very expensive ones (aobut 1K per window for a typical window) they are gross in terms of aesthetics and architectural integrity - and even the best ones will not last as long as the originals.
We're very lucky in this area to have superb window restorers. I had mine rehabbed by windowrepair.com and recommend them highly.
Every property on the market in Cambridge: http://goo.gl/dmS8d
What you can expect depends a great deal on what kind of property you're looking at. For example, if you're looking at renovated condos or townhouses, you should expect to find new replacement windows that tilt out for easy cleaning - a renovation that doesn't upgrade the windows may not be a quality renovation. But if you're looking at a multifamily home that's been owned by one or two families during it entire lifetime, you may find windows that are 25-40 years old! In which case, James is right, changing the enitire frame of the window isn't likely to happen unless you do a gut renovation. They can be upgraded to be functional for comfortable living though.
Generally speaking, windows more than 15 years old, are past their prime, and you should factor the cost of replacing them into your offer on the property.
You seem to be aware that windows are one of the big ticket items that effect the value and overall condition of your property - along with roof, siding, plumbing, electrical - if these things are ok, the property is in good shape, if they are not ok, they should be the first repairs you make on your new home.
One tip about windows, siding and roofs - usually one side of a house gets more weather than the others - so it is possible to do maintainance and repairs in stages if they are not in terrible condition - repair the side of the house that gets the most weather first, then each year, do another side - one way to budget for big ticket items.