After about a year, year and half, of playing I spent $20 since I liked the game and I am happy to contribute to something I use a lot. Other than that one time purchase, I used tapjoy to get a spirit mage. Took me about 2 years. Other people say they did it in about a year. Other people are probably a lot more dedicated than I.
I haven't really done much with the free gems lately, it is time consuming and much easier to get gems in the game now. But tapjoy is just a way of advertising. Through tapjoy, I found Dolphin Browser which was a great app up until an update or so ago when they started doing some questionable things.