How long does it take for one to get settled on a new saddle?
A 100 km ride is too much to do on a new saddle -- I'd start with 30 minutes rides, hour tops; not too hard. You also need breaks between rides -- riding 100 km then 30 km every day after will probably start you sore and keep you sore.

The saddle won't really change appreciably (break in) as you use it; break in is really for leather saddles which can break in like baseball gloves over time (and even then, theres only so much that can happen -- it either will fit or won't). Either you'll get used to it after a few rides or not. Many better bike shops have a return period for saddles that is long enough that you can probably get in 5-10 rides to get an idea if its comfy or not. 

It's a highly personal decision, with body type, bike type and clothing choices coming into play.

Also, make sure the saddle is properly adjusted -- this means adjusting angle, fore/aft position, height, etc; switching saddles isn't as simple as just putting the new saddle in exactly the same position as the old one. You may even end up adjusting the handle bars a bit; saddles can feel too narrow if you have to sit too far forward.