There are too many variables for there to be a "rule."
Heat, cold, playing surface, plastic type, type of objects struck, speed at which disc hits objects...it all depends.
Big Sky's correct in that there are a ton of variables (including those he mentioned), so it'd be virtually impossible to
quantify, but there are some guidelines which seem to hold true.
More premium plastics take longer to break in, but stay at the "sweet spot" longer.
Base line plastics tend to break in quicker, but lose their sweet spot quicker.
The geater the use and (perhaps more importantly) abuse, the faster a disc will change flight characteristics.
Putters will always break in faster because they're made to be much softer to grab chains, and rubbers tend to dry out more in sunlight.
I disagree with this statement.
Comparing drivers and putters
made of the same plastic, I think drivers usually break in faster (i.e. fewer throws - not necessarily # of rounds or days and weeks). This has more to do with the way the discs are used, rather than the type of disc. Drivers are thrown faster, hit more trees (and hit them harder), get tomawked, hit more asphalt on courses where OB roads can come into play, etc. and typically just take more abuse per throw than putters. Putters actually may get more use, but are typically thrown slower, hit fewer trees (and don't hit as hard when they do hit), so they evolve less per throw. However, since they're used on virtually every hole, that helps make up for their relatively gentle use compared to drivers.
It's not uncommon to see an experienced or pro player's bag with many drivers in premium plastics specifically because the those plastics age slower and can take punishment better than baseline plastics, and consistency is what pros and top ams covet most - they want to
know that disc will perform the same way every time, because that's what they're counting on.
Likewise, you'll find duplicate baseline putters in many top players bags, because they like them to break into a certain point, but since putters don't take that much abuse, they would take too long to break in if they were premium. Once they are well seasoned, as some on pointed out, they tend to stay there for a while because they aren't getting pounded as much, and if they're twins, they're sharing the load, and therefore aging even slower.
Don't get me wrong - many top players love certain drivers in baseline (Leopards and Tee Birds come to mind) and some folks love premium putters, and people certainly have preferences for certain molds in certain plastics, but as a rule, I'll stand behind what I said above.