It depends on the brand and the leather. I have a Bates and it took no time at all to break it in. What I would suggest is getting some Hydrophane Leather Dressing and oil it really well and work it into the flaps by bending them back and forth. Don't be afraid to really roll those flaps. I also use a nice conditioner called Passier Leaderbalsam which keeps the leather soft and supple too. Then just ride in it as much as you can.
Hydrophane is wonderful stuff! The Passier is actually my all time favorite conditioner, but it doesn't get deep down into the fibers like a good oiling will. A new saddle needs moisture to get soft and flexible. Some new saddles come with a waxy finish and you have to give it a good scrub before the oil can really soak into it.
And the more you ride, the softer it'll get. :) collegiates tend to break in nicely.
The way most saddles are tanned these days, you don't need to use a huge amount of oil for general cleaning. If you have a newer saddle, you'll only need it to oil it a few times a year.
For breaking in a saddle, try this. Clean it really well, then let it sit for an hour. Most new saddles are coated in a protective wax that needs to be cleaned off before use. Once it is completely dry, put a light coat of oil on it. Don't use too much, or let it drip or run down the sides as this will stain the leather. Put the saddle in a black, airtight trash bag and place in a warm room ( make sure it is DRY heat, not humid heat or it will mold!). Don't place it in front of any direct heat sources, such as a heater either. Leave in the trashbag for 24-48 hours. The leather will absorb the oil and be unbelievably soft and supple. My parents used to own a tack shop, and we used to use this technique whenever we had an old, dry, cracked saddle on consignment. It would make them like new again!
Most saddlemakers recommend riding in the saddle for about a week without any saddle pads - this allows the leather and flocking to conform to your horse's back. If you feel the need to use a pad to keep your saddle clean, use a very thin baby pad.