I've found getting my villagers to Master in Building and Healing takes the longest. I've found that it's best to start teens on it when they have no other skills, let them get to master, then put them on Research, then lastly Farming. I know that when I tried to finish off building skill on one of mine that had Farming and Research, she has never stayed on task long enough to Master it.
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2X Intel Pentium4 CPUs, 3.40 GHz
2 GB RAM
232GB-NTFS & 180GB-FAT32
Windows XP Home SP2
DirectX 9.0c (4.09.0000.0904)
NVIDIA GeForce 7600 GS 512.0 MB
Realtek AC97 Audio