My guess, and its just a guess, is that you'll follow a similar pattern to your dad. I am following a similar pattern to my dad, but I'm about 5 years ahead of him. He started thinning around 30, I'm 24. It sucks, but he's almost 50 now, and still has hair on his whole head, even though its pretty thin on top, so I'm keeping some hope of maintaining well into my 30s, but I've got a few genuine baldies (NW6's and 7's) in my family tree, so I might get the genetic shaft.
If you're worried that you will get worse than a Norwood 2 without meds, then by all means take them. As long as you don't get bad side effects, they probably aren't going to hurt anything but your wallet.
Generally you can tell how bad you'll get my looking at other balding family members. It's their crappy genes that are making you go bald.