Hi D. - With both my girls, the transition to whole milk lasted a few weeks before the were drinking only the milk. Some people suggest you can mix the milk and formula so that over time the milk pretty much replaces the formula - like starting out with 1 part milk and 2 parts formula, then half and half, etc. I actually didn't do this, rather, I started by introducing milk at meal times, and I kept the formula feed for first thing in the morning, before nap time, in the afternoon, and before bed. My girls drank milk with breakfast, lunch, and dinner. This way, if they didn't take to it, I knew they were getting their nutritional needs met by the bottles throughout the day. I found that it did take some time, but they slowly began to prefer the milk, so I began to spread it out to first thing in the morning and before bed. After a while, it was all milk, and around 11-12 months, I found with both girls that they gave up the big bottle when they woke up and before naps and bedtime anyway, so that wasn't an issue. My 13-month old drinks about 12-14 oz. of milk a day now, which my pediatrician says is great (she said anything over 10 oz. is fine, though obviously not too much either!!), and she drinks it basically all at mealtimes with the exception of a little right before bed as a wind down.
One last thing - I let my girls drink from their bottle at first, then started to put the milk in a sippy cup and began to slowly introduce the cup over the bottle in a way similar to giving milk over formula. You don't want to change both the milk AND the cup all at once. When I knew my girls were okay with the milk, I gave the cup more than the bottle, then slowly took the bottle away completely. My 13-month old is not totally drinking both water and milk out of a sippy cup. It can be confusing with all the changes - just take it slow and remember than it is a process. Good luck!!