i changed my rears last year in november and my fronts last year in around march, i had bought them at the same time it was just a matter of being stuffed to change them. they were all actually completely cactus when i took them out, but they were the originals, my car was built october 91.
a lot of people here will hammer you with the 'buy konis or at the absolute least kyb, anything else isnt worth it' crap.. but really you have to think about what you want to use your car for and what your price range is. its not worth getting the more expensive brands if you're just going to drive on the street, and gauranteed none of the people saying to get koni or kyb have ever been to the track anyway.
a set of monroe gas sports shocks will be a good improvement over the oiled up stockos, they're probably not as good as the konis or the kybs, but the difference wouldnt be all that much, afterall, there isnt going to be too many revolutionary advancements in how a gas strut is constructed from company to company.
i personally run monroe gas up front and gabriel gas in the rear. i've had no problems with either brands and my car handles pretty bloody well, both brands have a much firmer ride than the stock and oem oil replacement units, the gabriels are actually stiffer than the monroes. handling in my car has been simply excellent since installing them, but then i do have stiffer springs & swaybars as well.
as for when to replace them, if you push down on the back or front of your car and its easy to push down and generally squishy response, then you need to replace your struts.