The art is to make sure you burn a jar candle for long enough for the wax to hit the sides of the jar so it doesn't tunnel. Rosa says three hours. Now I find you need to burn them for at least four if not five hours to avoid that but perhaps that is just me. She also tells you to trim the wick before you light it to about a quarter of an inch and not to put the candle in a draught as it affects the flame and the burn.
This is all to get the full burn time. Now I do all this and use a shade on each candle and I NEVER get the full time so you could take the 150 hours etc with a pinch of salt. Don't get me wrong, I love YC but that is what I have found.
If you are unsure what you like then the samplers are a good bet but they do not give much of a perfume throw. You are best off going to a shop like Clinton's which sells YC and you can smell them before ordering online. Q also do a lot of sampler selections and the small jars do last for quite a while so are not the rip off they seem for such small items. I think if you like a certain YC perfume group, like fresh or floral for me, you will like most of the things in it. The wax melts (or tarts before Q decided to rebrand them..... Which CH implied was a matter of good taste!!!!!!) are also an excellent way of trying fragrances and can be bought cheaply but you do need to buy a tart burner so that is an added expense. Try also Yankee Direct and Yankee Doodle websites for good prices on individual items like burners and shades.