Several hundred years. Presumably we'll get a lot better at viewing exoplanets over the next hundred years, so by examining their atmospheres' we'll be able to find some juicy targets. Then it is just a matter of building the probe. I believe technology will continue to be developed at a very fast rate and 200 to 300 years from now we'll have some pretty crazy tech for space exploration capable of travelling at some modest but reasonable fraction of C.