The numbers are different for each advance, but if you have a focus on "windwheel", the mean time to happen (MTTH) is 500 months, but you just wait for tech spread, it's 2000 months assuming you border a province with that technology. It's seems that most other advances see a 3 to 4 fold change in MTTH, although other factors are involved.
I believe the effects are cumulative as well, so if you border two provinces with "windwheel", the MTTH is effectively 1000 months. So if you focus on advances that nobody nearby has, but wait for tech spread to bring in advances a lot of provinces nearby have, you won't see a big difference.