Questions re: hair coloring

I had been dying my dark-brown hair with a semi-permanent color to cover some grey, but noticed that it was getting too dark. So a few months ago I switched to a permanent color, which is slightly lighter and more golden-toned. However, at the time my hairdresser said she didn`t want put the new color over my old semi-permanent color (I had a lot left) because my already dark hair would just get darker. Instead she has been doing just the roots w/the perm. color, and then running this new color through the rest of my hair for only about 5 minutes.

Now I have noticed that new hair color closer to my scalp doesn`t quite blend with my darker sides and back. I guess this is because some old semi-permanent color is still left and she has only been putting the new lighter color over it for a few minutes each time.

When should she be able to put the new color over my ends for any longer than a few minutes without the risk of it becoming too dark? My objective is for it to blend and not get any darker. I haven`t had a semi-perm. color since December so I can`t believe there can be too much of the old color left - but I honestly can`t tell.

One more question - how often you run a permanent color all the way through the ends (as opposed to just doing the roots) without risking damage?

Thanks!