How long does it take?

Give them prezzies everyday like chocolate also talk to them everyday

If you give them presents they like (a music note will appear above their head) go onto the residents list and by them are hearts if two hearts are full the clour will change. Purple - 2 hearts Blue - 4 hearts ect.

Have fun

It all depends on how you treat the person, like giving them presents they really like will make them like you more, and talking to them everyday will also help. Giving them things they don't like can decrease affection, so be careful of that.

I had this problem, but I easily resolved it, what you need to do is wait for their Birthday's and give them gifts, any they like, and this immediatly increased FP more so then using a normal day. Festivals are also a great way of increasing FP with the characters, and some are more so then others. Example: The fish festival increases the hearts of the fishermen/women. So Lanna will have increased FP for a boy, and Denny for a girl.
Also, bring the dog if you have it (If you don't talk to Mirabelle and give her lumber- it's cheap and all you do is bang a stick with a axe, until she has 2 hearts. Maintain them, and get the animal barn and buy a cow at 5,000G wait until it is rainy or drizzly, go into the forest and this triggers an event which gets you the dog.) and talk to Vaughen who likes animals. This increases FP with him.
If you really wants lots of FP trigger the heart events, and this is an easy way to increase FP and change heart colours. Presents are the best way to get more hearts though.
-Hope it helped!
-Lucy