For example, I'm currently mining in a pool with one GPU at about 350 MHash/sec, and in the past 8 days have earned 2 bitcoins.  Projecting that out very roughly, if I were mining solo, that two weeks would represent 2/50 ths of the effort required to generate a block, which means I would on average generate one every 50 weeks.  Of course, by then the difficulty would have gone up dozens of times and I would have to be pretty lucky to generate one at all.